Hi John,
Locally (Mendix Studio Pro): Use the same admin password you set in the Administrator tab under the App Security page in Mendix Studio Pro. This password should work in the local environment.
For Published App (on licensed node): If you're running the app on a licensed node, go to the Details tab of the instance in Mendix Developer Portal. There, click on the ellipsis (three dots) icon, and you should see an option to Change Admin Password. From there, you can update the password for your admin user.
Once you've done this, you should be able to access the admin section after publishing, without relying on Demo Users.
I'm not sure whether you can set MxAdmin passw on freeplan or not.
You can also create accounts using after startup microflow - Make sure to check whether the account you want to create already exists if using this approach.