How to pass vendor_id to the SQL used in DFF - Valueset Default value in Value set

How do I pass vendor_id from PO headers UI to the SQL query in Default value
Requirement is to display sum of total PO amount created for particular supplier in PO page while creating new PO.
Step1: Creating DFF
Step2:creating valueset with Default SQL as below query. I need to know how to add a condition as supplier selected in UI page.
Currenty i am using below code but it is showing blank value.
/* SELECT SUM (CASE WHEN pla.ORDER_TYPE_LOOKUP_CODE ='FIXED PRICE'
THEN list_price
ELSE pla.quantity*pla.unit_price
END)
FROM po_lines_all pla,