1) Login to your Domains CPanel (You go to cPanel from WHM too)
2) Search for the icon named “Add-on Domains”, and click it.
3) Populate the directory name field.
4) Populate the password field with something which is hard to guess by anyone other than you. Make you of a combination of upper and lower case alphabets, numbers and special characters.
5) Click on Add Domain option
You can now upload your website files and folders to the specific add-on folder.