Frame-based URL Forwarding
Please visit the menu 'DNS' in 'My Domains' with the domain you want to add or modify an URL forward.
Now add a new record of type "URL-Forward", or click the "edit" symbol to change an existing one.
Click on "Options", and mark the checkbox 'keep the URL', if you want your original (sub-)domain ('www.your-domain.com') to remain visible in the browser's 'Location'-bar.
Please find more information in What is URL Forwarding (Web Forwarding)?