If you'd like the service selected on the Appointment form and/or Booking form to appear in your reminder messages, you can!
First, you'll want to ensure that all of the services you want available as an option are set up on the Settings > Services page. (For more details on how to properly set up Services, see the article "How do I create a Service?")
Then, to add the {{Service}} template tag to your reminder templates, you'll want to do the following:
Go to the Settings > Reminders page.
Hover your mouse cursor over the Template Tags section below your selected reminder template to make the template tags appear.
Click the {{Service}} template tag to include it in your reminder message.
Customize your reminder template with any additional tags or information you'd like the client to receive in their reminder message.
Click Save next to the reminder template.
Now any service selected in the Service dropdown menu on the Appointment form or Booking form will appear in your reminder messages when reminders are sent to your clients.