It is very simple and easy process. For adding addon domain in your cPanel you must login in your cPanel using your username and password at https://yourdomain.com/cpanel where yourdom ain.com is your domain name.
Now go to the Domains section and click on the Addon Domain available in Domains menu.
Enter your registered domain name in the relevant field and click on Add Domain button.
You have successfully completed the addon Domain procedure and now you can upload your website using the File Manager or by clicking on the highlighted link.