This guide shows how to create an addon domain in cPanel.
Once logged into cPanel, click the addon domains icon
Now you have accessed the addon domains page, you can set up additional domain names that point to sub domains of your account.
Now fill out the short form, once you fill the domain name box, the sub-domain/FTP username box will be automatically filled out for you.
You may need to tick the box "Create an FTP account associated with this Addon Domain"
Next, you will need to enter and confirm a password, followed by clicking "Add Domain"
The domain will now have been created!