When the song is playing tap on the repeat button until you see "1". Or create a playlist that only has that song and repeat the playlist.
When I click the repeat button I can only toggle twice. There comes no '1'. Im talking about the preview app though.
Will most of your just stop with "ugh, it doesn't have even this most basic thing".
*There is song repeat* in both Xbox Music and Groove on both phone and desktop.