Forwarding domains or using naked domain names
The procedure in Setting up a custom web domain for your website only works for subdomain web addresses where there is a prefix such as 'www'. So it would work for addresses like http://www.example.com or http://news.example.com but not http://example.com (sometimes known as a naked domain or root domain name).
There are technical reasons for this, but generally, it's also not a good idea to point an entire domain anyway. If you were using any other non-web services on on your domain (the most common non-web service being email), pointing the entire domain will break them and result in loss of service.
If it is important that your site can be reached through the naked domain name, then the recommended solution is to use http redirection, also known as web forwarding. You want to set up redirection (or web forwarding) to redirect the naked domain name to the 'www' domain name.
For example, you want to redirect any visitors from http://example.com to http://www.example.com where they can then see your ProsePoint Express site.
Many domain registrars offer http redirection as part of their service so it is convenient to configure it there.
Terminology differs depending on the registrar, but it is usually called web forwarding or domain forwarding (but without masking). If you are given an option to select the redirection type, the correct option is 301 or 'Moved Permanently'.
(Http redirection or web forwarding is also used if you want to redirect one domain to another, different, domain. The technical details are the same).
Web Redirection Services
If your domain registrar does not offer http redirection or web forwarding, then there are many third party services on the web which do. You could try http://wwwizer.com or search the Internet for other providers.