@woolfie Sonos's API actually doesn't allow third party apps (like Flic) to start playlists from third party providers like Spotify. The way we do it on Android is a bit hacky.
The solution is to instead use Sonos playlists, which can be controlled by third party apps. A Sonos playlist can be created in the Sonos app and can contain music tracks not only from Spotify, but from for example Soundcloud as well. Once you have created your playlists in the Sonos app, they will be visible in the Flic app under the Playlist option when configuring the Sonos action. You will then be able to use Flic as a trigger to start such a playlist.