The plugins and the command line cannot sign in with a browser, so they borrow your sign-in: the app shows a short code, you approve it here, and it may then share screenshots or submit benchmark results in your name until you disconnect it.
Sign in first
Everything new on this site comes from a signed-in GitHub or XIVAuth account. Your code is kept while you sign in.
Signing in with FFXIV needs an XIVAuth account and shares no character.
Link an app
Only approve a code you asked an app for yourself, just now. If someone sent you this code or this link, say no: approving lets whoever holds the app post in your name.