Heya @Barefootnd,
Thanks for writing in with this question!
Right now, Square Appointment bookings cannot be locked with a pin. If you want to make sure that only existing customers' bookings get approved, you can change your settings via Appointments > Settings > Online Booking Preferences:
When this is turned on, you'll receive a message to decide whether you want to accept or decline a given appointment.
Please let us know if this could be a workaround for you!
