Hello,
I just tested this on a dedicated IP. If you use these settings, you do not have to pass the @domain.com portion for the username:
Host: dedicatedIP#
Username: createdftpuser
Password: passwordofuser
Please replace dedicatedIP# with the dedicated IP, createdftpuser with the FTP user created in cPanel > FTP Accounts area, and passwordofuser with the password for that created FTP user. Only dedicated IPs will allow this to be done. You must use the dedicated IP as the Host for this connection to work.
Thanks!