If you get the post flight survey (VOC - Voice of Customer) just fill it in and submit. Else use the feedback form on the website.

I wouldn't be a happy camper either to be honest. We had a toilet without light on a MUC-SIN flight in J (the one in front of my 11A seat) which wasn't blocked per se, but most folks didn't use it due to that. Meant I never had to wait and just placed my phone with flash light on on one of the fresh hand towels on the counter and voila it was fine .