Well here's the error message the person is getting. And it's not working fine with the www.
Turns out that when adding that - it's actually just pointing to the main domain. So it acts as a parked domain.
Addon Domain Additions
could not be modified because it does not resolve to a valid ip address. Please ask the system admin to check their resolv.conf and their dns trust relationship Ignore any messages of success this can only result in failure!
Nameserver ips for addondomain.com are: 18.104.22.168,22.214.171.124
Created DNS entry for addondomain.com
Setup Mail forward addondomain.com -> maindomain.com
Setup addondomain.com OK
Adding httpd.conf entry for addondomain.com to point to maindomain.com
addondomain.com has been setup. It can be accessed via the subdomain /addondomain.maindomain.com. FTP access has been granted with the username /addondomain and the password ********