Hi @mmn
Thanks for the question.
Lets assume you are on the edit page for a person, and you want to select 1 event and then 1 Appointment.
You can use ACF to create the Event field (post object field). This will show all Events posts and allow 1 selection.
Next, you need to update / populate the Appointment select field. I would create this as a select field with no choices, and rely on JS to populate is after an Event has been selected.
You could write a jQuery event for the Appointment Change, and then get the Appointments via AJAX, then just update the Appointments select DOM object with the results.
Good luck!
Thanks
E
