How to check if a user fills a header text in billing invoice

Hi everybody and thanks in advance.

My consultant wants to oblige all users to fill a header text when they create the billing invoice (vf01).

I was looking for several user exits when I can manage this requisite.

My problem is that when the user creates the invoice and push save button, I try to look for in the table STXH if the user created the header text. But I have not found anything, because the invoice has not created yet.

Only when the billing invoice has saved, the header text is stored in the table STXH.

I has been trying to find a structure wich contains the header data like xvbrk contains the data about type of invoice, etc.

I have studied this user exits:

userexit_number_range (module pool saplv60A, program rv60afzz)

userexit_account_prep_komkcv (same module)

userexit_account_prep_kompcv (same module)

userexit_fill_vbrk_vbrp (same module)

v60A0001 (Customer functions in the billing document).

My question is: In which structure is the header text of billing invoice manage before push the save button?

If I knew where it is, I could manage the data and check if the user filled the header text.

Thank you very much, and sorry for my pour English.