Hi,
when you create your trial instance with the MDC feature you will get a complete tenant database for which you have to specify the password for the SYSTEM user. So initially the SYSTEM user is the only one you have, so this is the one you need to use for your first log on.
From this point on it's completely up to you: you can use the SYSTEM user to create new DB users within your instance and grant them the privileges they require.
Best regards,
Oliver