CNAME to an external subdomain? [closed]
If the IIS site will accept the incoming Hostname myhostname.mydomain.com, then perhaps the simplest solution is to skip the CNAME record and create an A record myhostname.mydomain.com -> [IIS server's IP address].
If you're using Apache, you can do the redirect you're asking about with the rewrite functionality in your site configuration file, or in the .htaccess file.