In an appointment reminder, you may want to include the "Add This To Your Calendar" option to make it easy for your customers to simply put the event in their personal calendar. This can be done through the Custom Values in the email builder with a few easy steps.
Step 1: In your system, navigate to your "Message Templates". Either click "Add New" or click the edit icon on an existing template.
Step 2: In the pop up email/SMS builder window, there is a dropdown of "Custom Values". Choose Appointment > Add to Google Calendar OR Add to iCal/Outlook.
Step 3: Click save at the bottom of the builder screen.