How to manage the Host directive in the robots.txt file
To specify which page of your website is considered a primary mirror, the search engines, you have to write in this selected domain in the Host directive in the robots.txt.
To do this, in the website management system, go to "Options" - "Domain and e-mail @yourdomain":
If your website hosts domains, then you will see a "Connected domains" table. Click on the "Set as main domain" hyperlink opposite the necessary domain.