Log in with your full email address and the usual password. Roundcube works more like a mac email program than a normal web app: filing is by drag and drop and you double click to open a message.
This is a bit more old school, but it’s the easiest way to add and amend email accounts, forwards and lists. Log in with the username ‘postmaster’ and your domain, using the password you will have been provided with if you’re allowed to do that sort of thing.
If you have any problems with email, please let will know.