You can't add them to the game proper but should be able to use the Analysis Board & Editor. You'll need to post that to a blog or forum to save it or copy the PGN somewhere.
But there are a ton of tools out there too for what you want.
https://www.chess.com/analysis-board-editor
Title says it all. I'm having trouble finding other software to annotate games. So maybe I'll just use the on site tools and post here when I want to share a game. But how does it work? Where do I go to add notes and variations to a completed game? Thanks for any help.