When edditting a demo user, you can copy the demo users password to your clipboard
The MxAdmin account isn't created in a sandbox/free environment.
As work around; You can create an after start-up microflow which creates this Admin user for you
just to point out something that I often overllok:
In the free sandbox app you can use the demo accounts,
What I normally do is put the security to Off and deploy to the sandbox. Now it is possible to add new accounts en give them the rights needed.
After that I redeploy with full security.
Your app is vulnerable for a few minutes, keep that in mind when using this method.