If you say "play X by Y in Poweramp" it will do it, and it seems to remember so you don't have to specify Poweramp most of the time. It tries to open Samsung music when it mishears me and I don't have the song.
But it doesn't start playing automatically most of the time. And if it was already playing something it usually won't switch songs, even though Google assistant pulls up the artist in the preview even when I don't specify the artist. Seems like Poweramp already has some sort of integration, it just needs some work.