First login to your cpanel page and after getting logged in go to the domains section and click on the add-on domain icon.

By clicking on the add-on button you will be redirected to a new page where you can register your new add-on domain within some time.

Here you can fill the new domain which you want to add-on like the following

After filling all the valid details click on the add domain button.

Your add-on domain will be created and you can access that domain within no time.