Log onto your account.
On your Dashboard, click onto 'Services' at the top of the screen. From the list of options available, click 'My Services'.
Find the service you want to create an Addon Domain for and click the three dots beside it on the right-hand side.
Another list of suggestions will pop up. Click on 'View Details'.
Click on 'Addon Domains' on the left-hand side of the webpage. From here, you can check any addon domains created or create a new one by clicking on 'Create Addon Domain' on the right-hand side.