Pavan,
I agree with Jurgen, in the image of the proforma invoice you have indicated PO number (which is blank).
Please check if the PO number is to be extracted from the sales order (VBAK-BSTNK)?
(and not from EKKO)
The PO number from sales order can be populated in the billing document (VBRK-XBLNR or ZUONR) and this inturn can be populated in your output.
TW