An addon domain is a fully functional domain that works from a folder in the FTP of your main site. These types of domains allow all the same characteristics as the domain owner of the account: subdomains, emails, forwarding, autocontest, etc. can be created.
All additional domains in an account must be handled from the same CPanel control panel.
You can not create an additional domain if your domain has not fully propagated. Before you change the DNS of the site you are trying to add, you should wait 24-48 hours before trying to create your domain addon. There are 3 fields that CPanel requests when creating an additional domain.
1. "New domain name:" Here you must go to the new domain.com. Do not put www in the name.
2. "User and subdomain name:" This will be the folder that CPanel will create in your main account. BE SURE that the name you give to the folder is not being used by another folder. Example:
If you want to give the username chris, there can be a folder in your account called chris. If it already exists, and you still try to create the user for the domain addon with the name chris, many things will be spoiled in the configuration.
3. "Password:" Enter the password you want here.