Until now, there hasn’t been a standard method to discover a website’s Onion site from the main website. That changed with the new release of Tor Browser version 9.5. It introduced a new Onion-Location HTTP response header. Websites can include this response header and have it include a link to the Onion site. The header should including the Onion site origin, plus the same path and query parameter as the current request.
The Tor Browser will display a “.onion available” button in the address field for encrypted websites (HTTPS) that includes the new header. Clicking it will redirect the user to the same webpage on the Onion site. The first time a user encounters this button, they’re also prompted if they want to automatically be redirected to Onion sites when one is available.