An addon domain is an additional domain that the system stores as a subdomain of your main site. Use addon domains to host additional domains on your account.
Before adding the Addon domain, some hosting providers allow you to add a domain only after updating the domain nameservers with the hosting provider’s nameserver. So please update the domain nameserver before trying to add the Addon domain.
Once the domain is correctly pointed using the nameservers, you can go the the Addon domain tool in the cPanel.
Inside the Addon domain tool, you can see 3 text boxes, where the first one is New Domain Name. You must fill this text box correctly without any space or anything in the format domainname.tld
Eg: google.com / google.net / google.co.in / google.in etc
Just below that, there is Subdomain and Document Root which will be generated automatically by the server. If you need to change it, you can type the requirement there and thus you can change the subdomain name and also the document root.