You have to run the application with your project security level at Off.
Then you will access the app without login and you’ll be able create a MxAdmin user on the the app.
Then you change your project security level and run to cloud again.
You will have now one access login via MxAdmin who is the first user :)
Hope it helps you.
When you use Sandbox, MxAdmin user also will not be created. Not sure if are facing this issue.
You can create users needed with ASU’s. Probably the Demo users might still work, if you have it enabled.
When you use paid Mendix cloud, then you will be given an option to use MxAdmin user and you can override password. For sandbox MxAdmin is not an option.
Hope this helps.