pods form and relationship field

Hi
i have a CPT called “events” with a Relationship field to users called “participants.”
Now i want my users to “sign up” for an event by submitting a pods form.
If they are already signed up (they are listed as participants) – they should again unsign with the same form (different label though).

is this possible? (they don´t have permission to edit this cpt)

best regards,
Andreas