https://username.github.io/repository-name/
_github-pages-challenge-username.www6 = record name
"123456123456123456123456" = value
Creating the site for an organisation involves an additional step. When you create the custom domain you do so under the organisation settings and not the user profile settings. Enter the organisation name in YouDoCMS - pages - settings. This will generate a different CNAME record using the organisation rather than the user profile. Lastly attach the custom domain to the organisation site as described above.