To add appointment details to your Encounter, Letter or Form templates, click on Insert Variable and then on the preset appointment variables (Appointment Practitioner, Date or Start Time) as shown below:

This preset data variable will appears as a drop-down in the encounter, letter, or form, as shown below. 

Note: You only have to select the relevant appointment details once. The rest of the appointment data variables will auto-populate in the specific template based on your choice.