I created an additional ftp account - how should I log in?

By creating an ftp account from cPanel, you create a login and password, which allows you to set the login to the additional directory on the server (by default, a directory with the name identical to the name of the ftp account is created). You can also change the directory to which the account refers when creating an ftp account (in particular, it can be the server's root directory).

The login of an additional ftp account is always "login@customer-domain.eu", and not the account name itself (ie the login to ftp contains the domain)