The training API works with the training/workouts section of Garmin connect so as long as you can push workouts from connect to your watch via either Garmin connect mobile or garmin express on pc.
Assuming it’s similar to fenix 5 software then to load a workout hit start → bike (or bike indoors) → options → my workouts and you will have to figure it from there as I have nothing loaded on the watch right now to see the rest of the steps.