You have to use the OrderRels endpoint
/{currentCompany}/Erp.BO.SalesOrderSvc/OrderRels
With this input , and it expects the Header and Line to Already Exist.
{
"Company": "string",
"OrderNum": 0,
"OrderLine": 0,
"OrderRelNum": 0,
"LineType": "string",
"ReqDate": "2022-03-08T18:50:50.084Z",
"OurReqQty": 0,
"ShipToNum": "string",
"ShipViaCode": "string",
"OpenRelease": true,
"FirmRelease": true,
"Make": true,
"OurJobQty": 0,
"OurJobShippedQty": 0,
"VoidRelease": true,
"OurStockQty": 0,
"WarehouseCode": "string",
"OurStockShippedQty": 0,
"PartNum": "string",
"RevisionNum": "string",
"TaxExempt": "string",
"ShpConNum": 0,
"NeedByDate": "2022-03-08T18:50:50.084Z",
"Reference": "string",
"Plant": "string",
"SellingReqQty": 0,
"SellingJobQty": 0,
"SellingJobShippedQty": 0,
"SellingStockQty": 0,
"SellingStockShippedQty": 0,
"SelectForPicking": true,
"StagingWarehouseCode": "string",
"StagingBinNum": "string",
"PickError": "string",
"CumeQty": 0,
"CumeDate": "2022-03-08T18:50:50.084Z",
"Linked": true,
"ICPONum": 0,
"ICPOLine": 0,
"ICPORelNum": 0,
"ExtCompany": "string",
"ScheduleNumber": "string",
"MarkForNum": "string",
"DropShipName": "string",
"RAN": "string",
"DemandReference": "string",
"DemandSchedRejected": true,
"DatePickTicketPrinted": "2022-03-08T18:50:50.084Z",
"ResDelivery": true,
"SatDelivery": true,
"SatPickup": true,
"VerbalConf": true,
"Hazmat": true,
"DocOnly": true,
"RefNotes": "string",
"ApplyChrg": true,
"ChrgAmount": 0,
"COD": true,
"CODFreight": true,
"CODCheck": true,
"CODAmount": 0,
"GroundType": "string",
"NotifyFlag": true,
"NotifyEMail": "string",
"DeclaredIns": true,
"DeclaredAmt": 0,
"ServSatDelivery": true,
"ServSatPickup": true,
"ServSignature": true,
"ServAlert": true,
"ServPOD": true,
"ServAOD": true,
"ServHomeDel": true,
"DeliveryType": "string",
"ServDeliveryDate": "2022-03-08T18:50:50.084Z",
"ServPhone": "string",
"ServInstruct": "string",
"ServRelease": true,
"ServAuthNum": "string",
"ServRef1": "string",
"ServRef2": "string",
"ServRef3": "string",
"ServRef4": "string",
"ServRef5": "string",
"OverrideCarrier": true,
"OverrideService": true,
"DockingStation": "string",
"Location": "string",
"TransportID": "string",
"ShipbyTime": 0,
"ChangedBy": "string",
"ChangeDate": "2022-03-08T18:50:50.084Z",
"ChangeTime": 0,
"TaxConnectCalc": true,
"GetDfltTaxIds": true,
"TaxRegionCode": "string",
"UseOTS": true,
"OTSName": "string",
"OTSAddress1": "string",
"OTSAddress2": "string",
"OTSAddress3": "string",
"OTSCity": "string",
"OTSState": "string",
"OTSZIP": "string",
"OTSResaleID": "string",
"OTSContact": "string",
"OTSFaxNum": "string",
"OTSPhoneNum": "string",
"OTSCountryNum": 0,
"SubShipTo": "string",
"ShipRouting": "string",
"BuyToOrder": true,
"VendorNum": 0,
"PurPoint": "string",
"DropShip": true,
"PONum": 0,
"POLine": 0,
"PORelNum": 0,
"OpenOrder": true,
"OTSSaveAs": "string",
"OTSSaveCustID": "string",
"OTSCustSaved": true,
"ShipToCustNum": 0,
"IUM": "string",
"SalesUM": "string",
"RelStatus": "string",
"ComplianceMsg": "string",
"PrevSellQty": 0,
"PrevPartNum": "string",
"PrevXPartNum": "string",
"PrevNeedByDate": "2022-03-08T18:50:50.084Z",
"PrevReqDate": "2022-03-08T18:50:50.084Z",
"PrevShipToNum": "string",
"MFCustNum": 0,
"UseOTMF": true,
"OTMFName": "string",
"OTMFAddress1": "string",
"OTMFAddress2": "string",
"OTMFAddress3": "string",
"OTMFCity": "string",
"OTMFState": "string",
"OTMFZIP": "string",
"OTMFContact": "string",
"OTMFFaxNum": "string",
"OTMFPhoneNum": "string",
"OTMFCountryNum": 0,
"ECCPlant": "string",
"WIOrderLine": "string",
"WIOrder": "string",
"WebSKU": "string",
"ShipOvers": true,
"WIItemPrice": 0,
"WIItemShipCost": 0,
"SysRevID": 0,
"SysRowID": "00000000-0000-0000-0000-000000000000",
"EntityUseCode": "string",
"PhaseID": "string",
"OTSShipToNum": "string",
"WasRecInvoiced": true,
"ContractID": "string",
"LinkToContract": true,
"ReadyToFulfill": true,
"OTSEMailAddress": "string",
"AttributeSetID": 0,
"NumberOfPieces": 0,
"NumberOfPiecesUOM": "string",
"PlanningNumberOfPieces": 0,
"AvailableQuantity": 0,
"BuyOverride": true,
"CreditLimitMessage": "string",
"CreditLimitSource": "string",
"CurrencyCode": "string",
"CustAllowOTS": true,
"CustomerAllowShipTo3": true,
"CustomerCustID": "string",
"CustomerName": "string",
"DisablePlantWhse": true,
"DocSelfAssessTax": 0,
"DocTotalTax": 0,
"DocWithholdTax": 0,
"DropShipOverride": true,
"DspInvMeth": "string",
"DspRevMethod": "string",
"EnableBuyToOrder": true,
"EnableMake": true,
"EntryProcess": "string",
"ExistPOSugg": true,
"HdrOTS": true,
"IntExternalKey": "string",
"InvtyUOM": "string",
"KitDisable": true,
"LinkToPONum": true,
"MakeOverride": true,
"MarkForAddrFormatted": "string",
"MarkForAddrList": "string",
"MFCustID": "string",
"NoRelTaxRgnChange": true,
"NotCompliant": true,
"OnHandQuantity": 0,
"OTSSaved": true,
"OTSTaxRegionCode": "string",
"PartExists": true,
"PhaseWasRecInvoiced": true,
"ProjectID": "string",
"ReleaseStatus": "string",
"RemoveManAdTax": true,
"Rpt1SelfAssessTax": 0,
"Rpt1TotalTax": 0,
"Rpt1WithholdTax": 0,
"Rpt2SelfAssessTax": 0,
"Rpt2TotalTax": 0,
"Rpt2WithholdTax": 0,
"Rpt3SelfAssessTax": 0,
"Rpt3TotalTax": 0,
"Rpt3WithholdTax": 0,
"SalesOrderLinked": true,
"SelfAssessTax": 0,
"SellingFactor": 0,
"SellingFactorDirection": "string",
"ShipToAddressFormatted": "string",
"ShipToAddressList": "string",
"ShipToContactEMailAddress": "string",
"ShipToContactName": "string",
"ShipToSelected": true,
"SNEnable": true,
"ThisRelInvtyQty": 0,
"TotalJobStockShipped": 0,
"TotalTax": 0,
"UpdateMarkForRecords": true,
"VoidOrder": true,
"WithholdTax": 0,
"AllowTaxCodeUpd": true,
"EnableDynAttrButton": true,
"AttributeMismatch": true,
"BitFlag": 0,
"DynAttrValueSetShortDescription": "string",
"DynAttrValueSetDescription": "string",
"MFCustNumInactive": true,
"OrderLineLineDesc": "string",
"OrderNumCurrencyCode": "string",
"OrderNumCardMemberName": "string",
"OTMFCountryDescription": "string",
"OTSCntryDescription": "string",
"PartNumTrackInventoryAttributes": true,
"PartNumAttrClassID": "string",
"PartNumPricePerCode": "string",
"PartNumTrackDimension": true,
"PartNumIUM": "string",
"PartNumTrackSerialNum": true,
"PartNumPartDescription": "string",
"PartNumTrackLots": true,
"PartNumSellingFactor": 0,
"PartNumSalesUM": "string",
"PlantName": "string",
"PurPointAddress3": "string",
"PurPointZip": "string",
"PurPointName": "string",
"PurPointCountry": "string",
"PurPointAddress1": "string",
"PurPointState": "string",
"PurPointCity": "string",
"PurPointAddress2": "string",
"PurPointPrimPCon": 0,
"ShipViaCodeWebDesc": "string",
"ShipViaCodeDescription": "string",
"TaxRegionCodeDescription": "string",
"TPShipToName": "string",
"TPShipToBTName": "string",
"TPShipToCustID": "string",
"VendorNumState": "string",
"VendorNumZIP": "string",
"VendorNumName": "string",
"VendorNumAddress2": "string",
"VendorNumCountry": "string",
"VendorNumCurrencyCode": "string",
"VendorNumCity": "string",
"VendorNumAddress3": "string",
"VendorNumVendorID": "string",
"VendorNumDefaultFOB": "string",
"VendorNumTermsCode": "string",
"VendorNumAddress1": "string",
"WarehouseCodeDescription": "string",
"RowMod": "string",
"UD_SysRevID": "string",
"sgcCloseReason_c": "string",
"ShipRef1_c": "string",
"ShipRef2_c": "string",
"OrderRelTaxes": [
{
"Company": "string",
"TaxCode": "string",
"ReportableAmt": 0,
"DocReportableAmt": 0,
"TaxableAmt": 0,
"DocTaxableAmt": 0,
"Percent": 0,
"TaxAmt": 0,
"DocTaxAmt": 0,
"Manual": true,
"ChangedBy": "string",
"ChangeDate": "2022-03-08T18:50:50.084Z",
"ChangeTime": 0,
"ReverseCharge": true,
"OrderNum": 0,
"OrderLine": 0,
"OrderRelNum": 0,
"Discount": 0,
"DocDiscount": 0,
"Rpt1Discount": 0,
"Rpt2Discount": 0,
"Rpt3Discount": 0,
"Rpt1ReportableAmt": 0,
"Rpt2ReportableAmt": 0,
"Rpt3ReportableAmt": 0,
"Rpt1TaxableAmt": 0,
"Rpt2TaxableAmt": 0,
"Rpt3TaxableAmt": 0,
"Rpt1TaxAmt": 0,
"Rpt2TaxAmt": 0,
"Rpt3TaxAmt": 0,
"RateCode": "string",
"CollectionType": 0,
"Timing": 0,
"ExemptType": 0,
"ExemptPercent": 0,
"ResolutionNum": "string",
"ResolutionDate": "2022-03-08T18:50:50.084Z",
"TaxRateDate": "2022-03-08T18:50:50.084Z",
"DefTaxableAmt": 0,
"DocDefTaxableAmt": 0,
"Rpt1DefTaxableAmt": 0,
"Rpt2DefTaxableAmt": 0,
"Rpt3DefTaxableAmt": 0,
"DefTaxAmt": 0,
"DocDefTaxAmt": 0,
"Rpt1DefTaxAmt": 0,
"Rpt2DefTaxAmt": 0,
"Rpt3DefTaxAmt": 0,
"ManAdd": true,
"DedTaxAmt": 0,
"DocDedTaxAmt": 0,
"Rpt1DedTaxAmt": 0,
"Rpt2DedTaxAmt": 0,
"Rpt3DedTaxAmt": 0,
"ECAcquisitionSeq": 0,
"FixedAmount": 0,
"DocFixedAmount": 0,
"Rpt1FixedAmount": 0,
"Rpt2FixedAmount": 0,
"Rpt3FixedAmount": 0,
"TextCode": "string",
"SysRevID": 0,
"SysRowID": "00000000-0000-0000-0000-000000000000",
"SalesTaxDescDescription": "string",
"DocDisplaySymbol": "string",
"CurrencySwitch": true,
"CurrencyCode": "string",
"CollectionTypeDescription": "string",
"DisplaySymbol": "string",
"NoChangeManual": true,
"EntryProcess": "string",
"BitFlag": 0,
"RateCodeDescDescription": "string",
"RowMod": "string"
}
]
}