wanted to display PO no of sales order in Invoice smartform

Dear Experts,

I want to make changes in smartforms. New requirement of client is to print the PO Number of sales order in the invoice.

But same smartform is used to display Sales order Invoice as well as STO performa Invoice.

And the previous invoice format was developed by someone else so its becoming difficult for me understand the programming logic.

Kindly guide me how to fetch the data and get it printed on same invoice format of smartforms, i.e. PO Number field value from Sales order and Your Reference field value from Purchase order.