Need to add Customer PO in invoice

Hi gurus,

I need to add a field to be printed on Invoices with the details of Customer's purchase order number, but is not showing up correct data.

I'm experimenting with 2 tables VBAK & VBKD and I'm selecting BSTNK and BSTKD fields from table. Both tables are transparent tables

So which table and field should I use in SAP script to get the Customer's purchase order number?

Thanks guys

Regards