In cPanel scroll down until you see the Domains section. Click on Addon Domains.
Enter the domain you want to add in the New Domain Name text box. cPanel will auto populate the Subdomain and Document Root boxes.
Click add domain.
Once your addon domain has been created you’ll get a success message. Click Go Back.
Scroll down in the Addon Domain window and you should see your new domain in the Addon Domain table.
If your dns has already been updated and taken affect then your addon domain should work pretty much instantaneously.