To setup a custom domain, go to the settings section of your hub and select Custom Domain from the left hand navigation. Note: You will need to be an administrator to access this feature and a premium subscription.
With a premium subscription you can setup your hub on your own domain. This allows you to create branded portals for your audiences and increase the SEO (search engine optimization) of your guides to point to your domain.
You can learn more about managing custom domains using this guide.