Hi Rihan,
Someone has listed quite a few exits for MB21. Refer Link below
HOW TO FIND THE SCREEN EXIT FOR MB21
BADI MB_RES_BAPI_DETAIL1 should work. Try to create an implementation and see if you hit this badi at SAVE in MB21.
If nothing works, then try enhancement implementation in standard screen of MB21 to pass the values of custom fields.
Regards
Sandy