Getting into the sandbox is an undocumented feature. You just use ‘1’ for local running app, you set the administrator password in sprintr.mendix.com for any other environment, but the sandbox… it is just not there. The easiest way is in studio pro opening the App => Security. Set ‘Demo users’ to ‘enable’ and add a demo_admin having Userrole Administrator.
Then deploy your app to the sandbox by clicking ‘Run’, log in using the demo-admin and create some useraccounts.
Adding an after startup microflow that checks if a MxAdmin account is there, and in case it’s not create it and commit it (with the password you want), makes sure you have an admin account in the sandbox environment.
Interesting to see the community suggesting these creative workarounds. Sandboxes were designed to not have MxAdmin user at all. Because user management is done in Mendix platform (Project > Team). The app owner (Techincal Contact) automatically gets the Administrator role if I recall :)