If you have a dedicated IP address, you can create a wildcard A record by setting the Host Name value to an * (asterisk).
When you do so, if the subdomain queried does not exist, the server will respond with the IP address specified in the Zone File Editor as the wildcard.
For example, if someone were to visit http://wildcard.mytestdomain.com, and the subdomain "wildcard" does not exist anywhere on the account, the visitor is redirected to the IP address you specified for the wildcard.
Wildcard DNS queries do not work with our Web Hosting (though they do work with Shared, cPanel, and Plesk). Find out what kind of hosting account you have.