EpiCombo filter

Hi, I am hoping someone can help me with a filter in a reason code combo box in the Issues Miscellaneous material form.
The combo box is bound to IR.ReasonCode and when I add the filter ReasonCode=’SU-S’ I get the expected results however I would like to filter on the Part.ClassID field that is also avalible in the same form. I tried this ReasonCode=’?[ClassID."]’ and this ReasonCode=’?[Part.ClassID"]’ buut with no luck. Any help would be appreciated.