http://protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7ozyd.onion/support/set-default-email-handler
Chrome, Edge Chromium, and Brave 1. When using Proton Mail in Chrome, Edge, or Brave, for the first time, a service handler icon will appear in the URL field. Click on the icon, select Allow and then Done . (new window) Proton Mail is now your default email handler in Chrome or other Chromium-based browser.