Copy/Paste for browsers games on Itch.io is a bit tricky. There's lots of security policies both on Itch and in your browser that will try to prevent an embedded app from using the clipboard (all Itch games are embedded in an iframe on the page). Basically, this is to prevent some nasty embedded advertisement from pulling information out of your clipboard without your consent. Unfortunately, this means that copy/paste doesn't work in Itch games either.

There are some workarounds I can try. If I can find something that works consistently, I will update the app and let you know!