If you go to Appointments > Online Booking > Settings > Customer Profile Fields , you can add a "Custom Field" when clients book online. Ours is "Preferred Technician(s)" with "No preference" and our tech's names as options. Clients can pick as many options as they like and our team can see who they prefer booking appts with.
I know it's not as clear as having a little icon with a heart to signify that they have a requested provider, but the custom field will pop up under their name when you click on their appt so it's fairly easy to check the info quickly.