You did it in Unity, right? Here is something that might help you. This is a comment from Vimlark's theme revelation video and it is gold:
Here are some common issues with Unity games: If you're using Unity and you upload your game, added the 50px to the frame height, and are still not seeing the fullscreen button, make sure you have enabled the correct player. Go to: Edit>Project Settings...>Player>WebGL Settings>Resolution and presentation. In there select the default WebGL Template. (This one includes the Fullscreen button.)
I'm getting an error trying to run my uploaded build on itch. (The error message may state something about compression.) Go to: Edit>Project Settings...>Player>Publishing Settings Under "Compression Format" select: "Disabled"
Don't know it any of that was the problem, it probably wasn't, but these are very good tips.