I've been searching online and I can't find anything that either says MUTV can be added to VM Flex, or any graphics/screenshots showing it as a Flex add-on.
I did however find an AI response (I usually ignore as they are usually incorrect) but the suggestion it made seems to make sense in a roundabout way, although I'm unsure if it's correct.
"for a full Stream-only household (no 360 box), MUTV and LFC TV might not be accessible due to IP licensing issues, requiring you to potentially add it via the official MUTV app or website for streaming on compatible devices. "
If that's the case then it would seem that one would have to switch from a Flex box contract to take out a TV package with a TV360 box (not available in a full fibre area) and if taking out multiroom then the channel could be added and also streamed via the IPTV box.
Or use a workaround by subscribing directly to MUTV and either access it via a browser on or the MUTV app which is available on firesticks, ios and android devices (including Android TV and Apple TV), Samsung Smart TVs, LG Smart TVs, Amazon Fire TV, Roku TV and Xbox. It can also be cast to Chromecast or Airplay connected TV's.
Due to broadcasting rights though MUTV (and Liverpool TV) cannot show live Men's matches during the season