As I can guess, you wanted to create a new sub-domain each time the new user is registered. This is what Web service providers sometimes do, to serve up the users no having and not purchasing their own domains. But you don't have control over the domain system and server DNS.
Here is the alternative approach:
You need to use some alternative naming of the URLs, not based on subdomains. Consider using, for example, URL rewrite
An Example of URL Rewriting With ASP.NET