I go to the "Site Settings" in the Google Search Console. I select the main domain (site.com - without www) and click on "Save".
In response, I receive a message:
The procedure for assigning a primary domain involves checking your ownership of the domain http://www.site.com/ . Verify domain ownership at http://www.site.com/ .
I do not understand how and where should I confirm it? The documentation did not find anything about this.