Lilhelplease

If anybody can help me figure out how to add a new custom mega form to a Incineroar, I would greatly appreciate it. I have made the art which I thought would be the hard part, but I can't figure out how to make it work in game. I have made an entry in montext.rb and the form appears with debug form switching but I cant make the mega evolution trigger in battle. I am attempting to use a move evolution like Rayquaza with Flareblitz. Any help?