Checkmate by Castling!!!
I finally got checkmate by castling! I only started trying to get the checkmate today too!
How I did it is I went to Play Online and then Custom Challenge. Then you go to the top right corner which features a chess board and an eye. If you click on it you will see open challenges.
And if you select a low rated player, capture all their pieces, don't castle, and probably tell them to not resign (unless you believe that's cheating), you need to set up your position without causing stalemate and castle to deliver the checkmate!
If you don't know the perfect position, there's an example in the game I played:
https://www.chess.com/game/live/168387341040?move=0
Have a nice day!