You cannot create an addon domain if your dns hasn't proprograted to the server already. So after you change the site's dns you are trying to add you will need to wait 24-48 hours before attempting to create your addon domain.
There are three fields cpanel asks for when creating an addon domain.

    1. "New Domain Name:"   You will put in the do not put www in the name!

    2. "Username/directory/subdomain Name:" This will be the folder cpanel creates in your main accounts ftp. MAKE SURE that the name you give it doesn't already have a folder. Example... If you want to give it the username ABC. You cannot have a folder in your account named ABC already. If you do and still try creating the username for the addon domain to be ABC you will mess a lot of things up!!!

    3. "Password:" fill in any password you want here.

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