Add extra domain in cPanel is quite easy. Just follow the steps below:
Step-1: Login to cPanel.
Step-2: Type domain at the top search bar in cPanel.
Step-3: Click on Addon Domains.
Step-4: Next page add your domain name without any www or http (like: yourdomain.com)
Step-5: Click anywhere on this page and the rest of data will filled automatically.
Step-6: Add Domain.
Read More: How to install free SSL certificate.