Pop up when the patient is trying to book an appointment via the portal account if the patient file is not linked to the patient portal

The client would like to prevent patients who created their portal account (but not linked to their patient file) to book an appointment by having a pop us displayed once they click on Book an appointment.

In the pop up, it should be said that the patient needs to contact the clinic (to finish linking the patient portal to their patient file) before booking an appointment.