How do I modify a field of BSEG in FB01? Substitution on GGB1 and BTE does not work.

I'm having occupation connected mounting a circumstantial worth connected BSEG-FDTAG successful FB01.

My logic successful GGB1 and BTE is 

BSEG-FDTAG = '20241212'.

But it doesn't look to instrumentality effect. I can't besides find a userexit for FB01 connected save