Found under: Settings > Song Actions > Link

Links can be used to give quick access to extra files related to a song, such as a YouTube clip, a YouTube Music clip, an audio file (mp3, midi, etc.), an online webpage or another document (e.g. a PDF).  When opening these links using the play or open button at the bottom of the popup window, your Android device will open them using your default applications.

Audio file links can also be set as the default pad for the song allowing you to play along with a backing track if desired.

You can assign a page button to give you quick access to the link page.