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If you have a puzzle pgn already from somewhere, then you can post it into the forums by using the "Insert chess game or diagram" button. It is the icon on the top left of the text box where you write your forum posts. It looks like a mini chess board. If you follow the directions there and paste a pgn into that, then you can set the diagram as a puzzle there too.
p.s. If done correctly, your post will have a big gray board icon with no pieces in your post. This is a placeholder. Just post your message as normal and when you post, then the placeholder will be converted to the chess diagram you entered.