Tell a client you are running late, or ask about parking, from the appointment itself — and keep the conversation where the studio can see it.
On My Appointments, tap "Message client" on the appointment. It appears on any appointment with a client attached.
Choose "Running late" (then pick 5, 10, 15 or 30 minutes), "I've arrived", or "Confirm parking / entry". The message is written for you, in your name — edit it if you like, or ignore the buttons and type your own.
The client gets an email and an app notification. Their reply comes back to the same conversation, so you do not need to give out your own phone number.