there is a special page that will hold all hooks and code example. I have started working on it. https://easy-appointments.net/predefined-selections-responsive/
I will add all callback there so you can extend EasyAppointments even more.
Once the appointment is confirmed maybe? I’m not sure how this works, though I’m a programmer I’m new at wordpress and PHP. I found out some form plugins sync submissions with google spreadsheets (1 way), and since the people I made the web for don’t know to use the backend that would be nice. Maybe you could add that to the extension plugin, or/and add the hook to the free version so that people make new plugins for you.