How to maintain the customised transaction code

Hello ,

My company has customised a transaction code for purchase requisition and i want to make the plant field as default in the transaction.

I maintained the field WRK in SU3 but it isn't working for the customised transaction.

Could you please sugeest as to how i can make the plant field default for the requisitioner .

Or should i take help from ABAP?

thanks.

Raghav