After login to Cpanel, look for an icon named “addon domains”. Addon domains are the additional domains that are created subsequent to the primary domain. It does not mean anything to you? I understand. It just means it is one another domain in your cpanel account
First of all you must have bought a domain name prior to this procedure. You can buy one at Namecheap
Type the full domain name in the field called “New domain name”.
Then click on Subdomain, no need to modify this field. In Cpanel for some reasons, it creates an access to your website using this subdomain and the primary domain as root domain: like yourdomain.primarydomain.com
Then, check that the name of the folder is ok and press “Add domain”. This will create the SSL certificate with autoSSL (if set up), the DNS records, the folder that you can access through FTP.