Hello Jiya,
Suppose the field name of your Target date is ZZTRGTDT, then:
Put this code in the include ZXWOCU07 of user-exit IWO10009.
CAUFVD_IMP-ZZTRGRTDT = CAUFVD_IMP-GLTRP.
CALL FUNCTION 'CO_IH_SET_HEADER'
EXPORTING
CAUFVD_IMP = CAUFVD_IMP.
PERFORM HEADER_UPDATE(SAPLCOIH).
When you create an Order with Basic Finish Date filled OR whenever you put the Basic Finish Date and Save the Order the Target Date field is automatically updated with the value of Basic Finish Date upon SAVE event of the Order. You can verify this by re-entering the Order ( IW33 / IW32).
Hope this serves your purpose.
Also have a look at this related document posted recently. How to default values to Notifications and Orders ?
Regards
KJogeswaraRao