Rate & Heart Songs on Apple TV

I just stumbled on this one, it's on the "in-song" menu.

To get to the in-song menu:

Go to the "Now Playing" screen (track art/details center of screen) then click the remote trackpad to "zoom out", then above the song there will be a small circle with three dots inside. Swipe up to access the circle, click the track pad and you should them see the "in-song" menu where you can find the "I love this song" option, alongside the other normal "start station", "add to playlist" options.

Hope that helps.