Once you have added the domain name in the Addon Domains section inside your cpanel. A directory will automatically be created inside of your /public_html directory.

Ex. The Addon domain is test123.com.
When you add it, it will show up as /public_html/test123. Everything should now be uploaded inside this directory for that addon domain name that was created.

Note: In order for this to function correctly and for the site to be viewed, the DNS must match as the primary domain names DNS.

How do I change permissions on folders and files?
1. Login in to “cPanel” at http://your-domain/cpanel
2. Click on File Manager
3. Select the file name to be changed
4. Look at the list of options on the top right side of the page
5. Choose ‘Change Permissions’
6. You’ll see options to either change the read-write-execute settings, or to edit the chmod numerical setting directly.
7. Make changes
8. Click Change