Depends on the device you own. Either yoga is available on it directly, in that case you add it like any other activity (see the user manual for your model if in doubts). Or, when it is not available, and your device supports the Connect IQ technology, then you can install a CIQ app for yoga, or a universal one (like the "Any Activity" or "Any Sport"). Or you just record the yoga session as for example "Cardio", and then change it to Yoga in Garmin Connect.
Calories are always counted using the heart rate (as long as the HR is available), so the type of the activity you select, is actually irrelevant - you can use any available one that you find suitable.
