Login to your cPanel and click Addon Domains, under Domains. There are three fields cPanel asks for when creating an Addon Domain.
1. New Domain Name: Enter the new domainname.com, but do not put http or www in the name.
2. Username/directory/subdomain Name: This will be the folder cPanel creates in your public_html folder. Normally, you want to ensure the name you provide doesn't already exist as a folder in public_html, but cPanel will automatically suggest the name and we recommend you stick with the suggestion.
3. Password: Fill in any password you want here. This password is used for an FTP account which is automatically created by cPanel. You likely will not ever need this password, so we suggest picking a random password and forgetting about it.
Your new domain is now successfully added. You can upload the website files to the specific addon folder which was created in the public_html folder.
Reminder: Your new domain name will not display your site until you set that domain's name servers. You must go to the company where you registered your domain, and change the name servers there.
How to Remove an Addon Domain
1. Login to your cPanel and click Addon Domains.
2. At the bottom, under Actions, click Remove.