"OK, Google" Command To Launch Music App
I have a Sony Phone with both Stock Music player and Google Play Music player. 


  This works for me, I'm not sure for Samsung Phones. But you can give it a try.



I manage to open my Stock Music player from Google Assistant by saying "open music from Sony".
To open the play music, i say "open music from Google" (or) "open play music" (The app name).