You'll need to have the ProWritingAid Chrome Extension installed and enabled: https://prowritingaid.com/en/App/ChromeExtension. You can either set it to run on All on sites, or Specific sites. If you choose specific sites, you’ll have to give permission to app.atticus.io.
At that point it should be working automatically. If you have longer chapters, it can take a bit of time to load but you should still see the small green/teal loading icon when you scroll to the very bottom of your chapter.