You should use
firstname.lastname@example.org as username and the random string as the password, we will improve our documentation to clarify it.
You said that using the random string you are not able to access, maybe you are accessing
YOUR_IP and click on Sign in, but on this page, you are asking for a customer account, not the administration account. Instead of that, you should access to
YOUR_IP/administration using the above credentials to enter as admin.
Please let us know if this solves the issue.
Carlos R. Hernández