What is an addon domain?
Addon domains are a type of virtual hosting that allow you to manage multiple domains from a single hosting account. Addon domains share the hosting account’s resources with the main domain.
To login to the cPanel for your addon domain, just login to your main cPanel. It is all managed under one control panel. The username and password can be found in your welcome email. You can upload the website files to the specific addon folder which was created in the public_html folder.
If you are not using WHM, To login to addon domain the username and password you will have to use is of the main domain in which you have created the addon domain.