for www subdomain thats done with Auto-Subdomain when you setup a website, or you can have *. for redirect all subdomains to the site root folder.
if you want to setup a subdomain called ICE for domain MYSITE.COM (ice.mydomain.com), you would go to SITES/SUBDOMAIN OF WEBSITE and then put:
Host -> ice
Domain -> mydomain.com
Redirect Type -> L
Redirect Path -> /ice/
Active -> check
Redirection Types - what the URL in web browser will be:
L = ice.mydomain.com => ice.mydomain.com
R = ice.mydomain.com => mydomain.com/ice
Then add the dns CNAME or A record for ICE, restart apache and it should work.