It is possible to use the Custom Question to trigger Auto Actions,
Trigger tab :-
Booking Field Does Not Contain = Custom Question X - (leave next field blank) then this auto action will only trigger if the Custom Question field is filled.
Message tab :-
Enter the text you need to send to the guest with/without a payment link if there is additional payment required.
== Option 3 - If you want to create a Special rate for guests to rebook ==