You can add a playlist from your music library that plays automatically when you start a workout in the Workout app on Apple Watch. Here’s how:
- Open the Apple Watch app on your iPhone.
- Tap My Watch, then tap Workout.
- Tap Workout Playlist, then choose a playlist.
The playlist is added to My Watch > Music in the Apple Watch app on your iPhone. You should note that a workout playlist won’t play if you’re currently listening to other music or audio.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)