Bigin & Booking. Associate the appointment with existing customers in bigin.

Bigin & Booking. Associate the appointment with existing customers in bigin.

I tried to change the stage of the pipeline associated to a existing contact after he book a appointment in Booking. I use flow to create a event in booking when a appointment is done. But....

How can I relate the appointment with the existing contact and create the event related to this contact. 

The purpose of the event is to be able to use it as a trigger in the stage automation to make it change.