How to exit in F-28

Hi

I have used exit U001 in GGB4 for F-28 to check the length of reference field (bkpf-xblnr) at Header Document

But when i save the document in F-28 it is not going to those lines of exit

Can anybody explain me how to use exits in ggb4

Thanx in adv.

Regards,

Deepak.