&VBAK-BSTNK& is not printing data in form

Hi

I have made some changes SAP script for the SD Invoice to show the Customer Purchase Order by putting a Window in form and the putting above code (&VBAK-BSTNK&) in Symbols and Command but none of them worked.

Do I need do something differnetly? Is there any other way to get customer's PO data details in Invoice?

Thanks for your help.

regards