Michael Potuck, 9to5Mac:
After bringing the long-awaited feature to iOS last fall, Spotify has updated its app today to take advantage of Siri support on Apple Watch in watchOS 6.
And:
Spotify mentions in the release notes to use Siri with the music service on Apple Watch users can say, “Hey Siri, Play music on Spotify,” or just add “on Spotify” to any voice command to play content.
Works pretty much everywhere for me now, except HomePod. For that, you’ll need to use AirPlay.