SAP Enterprise Adapter Parameters vs B2B Adapter Parms

We have a service on the b2b side of our integrations that is able to send an inbound IDOC (INVOIC) to SAP passing certain parameters like IDOCTYP(idoc type), MESTYP (message type), and MESCOD (message code). HOWEVER…on the Enterprise side the only parameters we can specify are Idoc type and Message Type. There is not place to pass the message code parm. Anyone run across this? The message code is also called the message variant in SAP, which is required for the certain type of INVOICE we are passing.

We are using 4.5 Enterprise with ALE Client intelligent adatper.