Joe:
I found this under taCreateManualCheckLineInsert:
UPRTRXCD
UPR transaction code; this element is optional if passing any of the transaction types listed, but users must pass a valid transaction code on any transaction type not listed:
4=Federal tax;
5=FICA Social Security tax;
6=FICA Medicare tax;
11=FICA Social Security/tips;
12=FICA Medicare/tips;
13=Federal tax/tips;
14=Uncollected FICA Social Security;
15=Uncollected FICA Medicare
Maybe that'll work.
The way that I deal with this issue is to do the transaction that I want to in GP manually, and trace it using SQL Profiler to see the tables involved. In this case, I think that it's the UPR10308. Look in there and get the code, and use that.
Does this help?