The best way to make a default template for your appointments and booking reminders is to make it the Main Template in your GoReminders account.
To set this up, please do the following:
Go to Settings > Reminders.
Make sure Main Template is selected in the Message Templates section.
Type or copy and paste your desired message into the Main Template text box to replace the existing reminder text.
Include any desired Template Tags in your new Main Template by clicking on them if you would like customized appointment information to appear in your reminder message such as the First Name, Date, Time, Location, Service, etc.
Click the Save button next to the template to keep any changes.
Once these steps are completed, just select the Main Template when creating your appointments on your GoReminders calendar so your desired reminder message is sent out as the preferred reminder. You can also select the Main Template as your Instant-book Appointment Message under Settings > Booking, if Instant Book is selected as your Booking Style.