During the installation process, you would of been asked to create a username and a password. This username and password is what is used as your administrator account for QuickBox.
As per the image below, during the installation process, QuickBox will install all of the dependencies and additional software it needs for it to run. Once it has finished, it will output the results and tell you the URL you need to use to access your dashboard.
So make a note of this screen before you reboot the server so you’ve got all of the login details, and all of the URLs etc that you will need to sign in.
All you need to do from here, is just go to the URL given, and then you can sign in like below.