If you have enabled Demo Users in your app security, you can use the demo_users configured inside your mendix studio pro.
At sprintr.mendix.com select your app and go to Environments. At the right of your environment click on details. There you will find a button on the right “Change Admin Password”.
MxAdmin has by default ‘1’ as a password when you run it locally. That is safe enough on your local computer but would be a security risk when used anywhere else. Usually, you will set up another user account as being your administrator.
For a more detailed explanation read this guide https://docs.mendix.com/refguide/administrator
See the documentation here: https://docs.mendix.com/refguide/administrator#user-name
In the free environment there is no MxAdmin role. Your own account has the administrator role so when you log in as yourself you should be able to do all the administrator stuff.