How to Make PO filed mandatory in invoice work bench(except few suppliers)

Good Morning

We are using R12.1.3

Per business requirement - finance department wants all AP invoices  should have  PO number ( PO number is mandatory for all AP invoices), but with exceptions  few (2 or 3) suppliers .

These  few suppliers are like LOCAL Supplier - not PO will be issued as these services are miscellaneous.

By Rationalization- we make  PO number filed  mandatory ( On New Form Instance), but we unable to give exceptions for  such LOCAL Suppliers)

Can you please  advice to complete  this need.

Thanks