Hi there!
Right now it's not super obvious to add songs to a playlist either from the Windows App or the phone. Both the apps have some real estate that they could use to simply add a button to 'Add song to playlist' without having to click on the easily missed ellipsis (three periods: '...') . Using other apps like Spotify, etc, they make it super easy to do this. Those apps music finding algorithm is garbage, but some aspects of their app are superior. I'm a huge Pandora fanboy though, but I don't use this much for playlists since it's just a slight bit more annoying to create them on Pandora than other apps.
Another way to do this would be to have a floating icon that stays on top of all the windows so you can easily add a song to a playlist when the app is open.
If you have any other questions, let me know and I can provide more info!
