I want to make it so I can send people to a subdomain without actually creating the subdomain in my account. Would this be able to be done with custom 404 pages? Or would I need some code for this? What code would I use if I wanted it to use the subdomain text on the site? Saying if the user went to mysite.com/ABCD it would display on my site as "Thanks for visiting ABCD". Or if the visitor were to go to mysite.com/jasfbhiweuhr it would display "Thanks for visiting jasfbhiweuhr". Just wondering if this would be easy or if I will need to go get someone to code for me.
simple html redirect? <meta http-equiv="REFRESH" content="0;url=http://www.the-domain-you-want-to-redirect-to.com"></HEAD>
Those aren't subdomains, they are subdirectories or subfolders. To accomplish what you want, you need to implement URL rewriting. If using Apache, you would need to modify your .htaccess file. If using IIS (ASP.NET), you would need to modify your global.asax. Do a Google search for URL rewriting for more information.
Here's a handy guide on rewriting URLs in .htaccess - even a simpleton like me can understand it. corz.org/serv/tricks/htaccess2.php
Your host must also support URL rewriting, it must be enabled in httpd.conf. I found a lot of hosts not enabling this.