To insert a game in a forum post just click on the chessboard at the top left of where you are posting. You need to paste in the pgn file. The pgn file can be obtained by going to your game (as I just did), clicking on download, and copying the pgn file.
Just playing some casual games very rustily, and met a Scandinavian player. Although this mate in <15 moves was due to mistakes and blunders from both sides, it was the most fun I had on chess.com for a while.
https://www.chess.com/live/game/1913261932
P.S, how do u show a board on the forums? Thx