How to copy business partner in VA01 automatically

Hi experts,

Here's the problem I'm facing:

When creating/changing sales order, if user input 'WE' partner in the sales order header, then should copy it as 'Z3' partner automatically. I tried coding in the user-exit 'USEREXIT_MOVE_FIELD_TO_VBAK' and then append to internal table XVBPA. I debuged the user-exit, although the data append to XVBPA successfully, but when back to screen, the record does not exit in the list of business parner. So I'm wondering whether the user-exit I used is not exact position or there might be some update flags should be set in XVBPA.

Thanks a lot!

Zhengning