Form Question - appending to default url when posting

Hi All
I'm trying to figure out how to do this:

I have a form where i want the user to enter their domain name in a form. The form then takes them to their webmail url on a new page.
The first part of the url will be the same, i.e. http:mail.
When the user enters their doamin i.e. mydomain.com i want the actual url to be http://mail.mydomain.com

Hope this makes sense and someone can help!

Thanks in advance
Andy

 

just do a string replace, and redirect
pretty simple stuff, no matter what language you use