Regarding copy control from quotation to contract?

Hi abapers ,

i got requirement regarding copy control in quotation user maintain PO number and PO Date at header and item level that data that data need to filled with PO number and PO Date fieds at header and item level. here already done one enhancement so in that i have writtend code below like this . data is populated correctly but lasty its clearing by standarad program

VBKD-BSTKD = CVBKD-BSTKD.
VBKD-BSTDK = CVBKD-BSTDK.

please help me how to update vbkd data in va42 from va21.

i have used exit

USEREXIT_MOVE_FIELD_TO_VBKD its not working .

below is the requirement from FD.

*(that field BSTKD & BSTDK get copy to subsequent sales documents

enhancement ZSD_ COPY_ Control needs to be amended at both header & item level.)