REST API ARInvoiceSvc error "Sales Order is required." trying to POST invoices

I am running into an issue trying to POST invoices to the Epicor REST API.
/api/v1/Erp.BO.ARInvoiceSvc/ARInvoices/

Here’s my payload:

{
    "Company": "TST",
    "InvoiceNum": 0,
    "OrderNum": 12221,
    "CustNum": "4262",
    "GroupID": "GRP1",
    "TermsCode": "PRE",
    "FiscalYear": 2023,
    "FiscalPeriod": 6,
    "FiscalCalendarID": "Main",
    "CustomerCustID": "CustN",
    "BTCustomerName": "Cust Name",
    "SoldToCustomerName": "Cust Name",
    "CustomerName": "Cust Name",
    "SoldToCustNum": "4244"
}

Here’s the error I’m getting:

{
    "HttpStatus": 400,
    "ReasonPhrase": "REST API Exception",
    "ErrorMessage": "Sales Order is required.",
    "ErrorType": "Ice.Common.BusinessObjectException",
    "ErrorDetails": [
        {
            "Message": "Sales Order is required.",
            "Type": "Error",
            "Table": "InvcHead",
            "Program": "Epicor.RESTApi.Common.dll",
            "Method": "ThrowUpdateExtException",
            "ColumnNumber": 17,
            "LineNumber": 27
        }
    ],
    "CorrelationId": "7fdb80b3-c096-4b7f-82b3-feba1dfcf128"
}

As you can see, I am passing the OrderNum (which refers to a sales order).

If anyone has run into this issue before and has some knowledge to share, I’d appreciate it.

Thank you!

For anyone that’s interested, here is the payload with the fields required to push an AR Invoice (InvcHead):

{
    "Company": "TST",
    "InvoiceNum": 0,
    "OrderNum": 12221,
    "InvoiceType": "SHP",
    "Plant": "Plant",
    "CustNum": "4262",
    "GroupID": "GRP1",
    "TermsCode": "PRE",
    "FiscalYear": 2023,
    "FiscalPeriod": 6,
    "FiscalCalendarID": "Main",
    "CustomerCustID": "CustN",
    "BTCustomerName": "Cust Name",
    "SoldToCustomerName": "Cust Name",
    "CustomerName": "Cust Name",
    "SoldToCustNum": 4244
}