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This doesn't seem to work

I've installed the script to Chrome, and am using TamperMonkey so Chrome won't disable it. I have the Agent installed and set to load on startup. And when I play something with Plex, it plays in Plex, not an external player. Is there something I'm supposed to do to make this work, or does it just not work?

There is a separate button you should click rather than the standard play one.