This is much harder than you would think. However, the difficulty of this task depends on how popular the YouTuber is.
Mega-popular: This is like Flamingo-famous (I’m going to be using Flamingo as the example). I bet he has thousands of people sending him emails every day, meaning he is tough to reach. Your email might not even get read, and has a high chance of never making it to Flamingo himself. Another thing you could try is to comment on his video and pray it gets enough likes.
Popular (500K subs): This is a bit easier, as your emails are much more likely to be read. If they have join on, you could join their game and ask to discuss with them. You might be able to offer payment for playing your game.