From My20i, head to Manage Hosting > Select Package > Domains. This area allows you to configure all domain names associated with that hosting package.
Domain mappings or aliases are simply additional domains that point to exactly the same files - an identical document root - as your primary domain name on the package. So, to alias or map a domain simply type it in the 'Domain' box.
Please note: don't include 'www' and leave the document root as the default public_html.