If you do not have a domain-only subscription or a linux subscription on the same account then you will have to contact our Support department in order to set up a forwarder.
If you want to set up an alias then add your new domain as normal under All Domains, set the Web Hosting to Website and set the "Location" to be the same folder you set for the domain you want this to alias (typically /domainname.com)
On our older Helm windows hosting plans:
Click on the domain you wish to add the alias to from within your hosting control panel (Helm) You should see an icon labelled "Domain Aliases". If you click on this icon you can add or delete domain pointers. NB: Please let our support team know that you wish to add domain pointers so that we may add the domain to our DNS