Updte ZUONR in document FN8X

Hi

I have to update the field assignment (ZUONR) when I post an accounting document in FN8X.

I was looking for information about it and I found documentation about the user exit: EXIT_SAPLFVN_BUS_OPERATION_102

The ABAP made a brake point in the INCLUDE ZXFVNBOU02 however the program didn't stop.

Somebody knows if this user exit is helpfull? I have to activated this user exit?  where?

Could you recommend  other way to update the field assignment?

Thanks

Best Regards.

Adriana Rivera