WHMCS, Clientexec, Blesta all create a random lowercase/numeric username for the accounts.
This also can be seen when a customer logs into said management system, and also has a reset password there as well.
Or the customer could open a support ticket asking what their username is.
You can also choose a custom un if you are creating the account manually, as long as it in no longer than 8 characters.
Usernames can only be lower case letters & numbers.