How should I enable specific users to enter a configuration mode on a Kiosk?

I am building an App which will be installed on a Kiosk, the Kiosk's sole purpose is to run this app and prevent users from doing anything else beside than use the app.

After installation only authorised users will be able to view existing configurations and modify them after entering their credentials, obviously I would not want members of the public to enter this mode or know about it's existence.

Currently I have a few options on my mind but not entirely satisfied with any:

  1. Upon entering a magic value, the configuration mode login wizard will appear.
  2. A hidden field on the screen, that will bring up the configuration mode login wizard upon being clicked.
  3. A button that will only appear on the splash screen.