As far as i see it is not possible to add customze registration form field to the ticket or am i wrong?
That is correct. I started some patches in Indico 2 times [1,2], but it is tricky and the changes need to wait for the planned ticket editor rewrite.
 Introduce a parameterized Badge placeholder by bpedersen2 · Pull Request #3164 · indico/indico · GitHub
 Ticket designer: Allow to pick any registration field · Issue #3146 · indico/indico · GitHub
thanks, that what i told my bosses before…