MB1B line item number check

Dear Experts,

I want to make the line item number and the PO number mandatory when an end user is using MB1B and movement type 541.

I mean when I go to MB1B, and select the from PO button, I want the system to make the PO number and Line item mandatory

I am doing this to for the subcontracting scenario.

Please tell me how it will happen

Burzes