When you add a domain as addon in your cPanel, you fill out some information to create an independent ftp account to access directly to this addon domain directory in your account.
This is not an information we know nor we record in our system.
If you lost these information, simply login in your cPanel,
go to the FTP accounts section and you will see all your existing ftp accounts, you can there change the password for the addon domain corresponding ftp account.
As an alternative you can simply use your main ftp account, the one is created automatically when we have created your hosting account.
This FTP account login credentials are the same as your cPanel access login information.
Using your main FTP account you have access to the complete public_html directory, then you can browse to your addon domain directory to update/delete/upload your files.