Its not clear what you actually want to do , As far as I understand MxAdmin login works for you in your natice App .
Did you use Account Overview in Responsive part of the app to create the User and try login ?
If yes and still the issue , then you need to design a custom logic as follows :
Use Non-Persistent entity in the login page to enter the user name and password .
Have the login Nanoflow to take this value and call Sign in JS and pass these two values ( Username and password)