Hi,
I would suggest to stick with standard option, since it can fulfill the question you raised. You can maintain the tax indicator for the account assignment category in OMKO. Now, create a condition record with KPMP-TAXIK field as explained in the doc: Defaulting Tax Code in PO - Condition Technique
If you still want to go for development, refer the settings mentioned in the note: 566002 - M/03: Can't use field PSTYP in the condition table
Regards,
AKPT