I've tried to do ajax/xhr calls to https://itch.io/api/1/me/me but then I get CORS issue. I made a manifest and added together with the files but it seem to completely ignore the manifest in the 'play in browser' mode.

Is there any way to get the visitors username for me to use in the game (scoreboard)? Without needing to make it into an app or place it on a server.

At the moment we don’t provide API for fetching user’s credentials directly in the browser right now. It’s something we do want to add in the future, but it will come with a permissions dialog to ensure that no info about the current account is leaked to the game.