Open the media player app (looks like a musical note) on the watch or add the media player tile to the watch. You can the control the music on the phone or the watch by making the appropriate selection.
Odd. If you have the companion apps on the watch, you should be able to control the phone from within the watch app. I don't use music apps, I just have my songs on my phone, so I guess that's what the media player looks for -- songs downloaded on the watch or phone.
But what is the point of a companion app if you cannot control the stream from one device to the other? I took Spotify off my watch, but I just turned Spotify on on my phone and was able to control it with the music player tile on the watch. The podcast I was playing showed right up and I was able to pause and backtrack. (I don't actually listen to that podcast, it was just the first thing on my front page).
