Next Reservation Reminders can be enabled for each individual Single or Group Service
From the menu tap Manage > Services
Select and tap on the relevant Service from the list
Scroll down to the Next Reservation Reminder section
Slide the button to the right to enable this function
Enter the amount of days after the Service that you wish the rebooking reminder to be sent
Customizing Next Reservation Reminder notifications can be done via your web app, see How do I customize rebooking and review reminders for a Service?