When you register a domain xyz.com, the domain key will work on more specific subdomains like www.xyz.com. However the inverse is not true. If you register www.xyz.com, the domain key will not work on xyz.com.
The solution to this is to transfer your domain to be xyz.com. You can do that from the ‘Cancel’ tab of the domain details page.
If you are registering a new domain, we always recommend registering the apex domain (e.g. xyz.com) so it can also be used on subdomains. The exception to this is when your site is hosted on another service that uses a subdomain to redirect to your.
Exceptions: My site is hosted on herokuapp.com: Using the full subdomain is appropriate here. My site is always available on
www.xyz.com. If your site has a url rewrite from xyz.com ->
www.xyz.com or these domains point to different sites, then of course it makes sense to register the subdomain.
If you have additional questions, reach out to us at support@cloudsponge.com