How use ANDROID NAVIGATION BUTTONS

Hello everybody. I'm trying to figure out when the user clicks the navigation buttons on Android devices (Unity 5.2). But I have nothing. PLEASE HELP ME TO UNDERSTAND WHY unit can not do that. What should I do, that we could finally clicked DETERMINE THE "Menu | Home | Back." All I Found in the vast almighty Google is:

 if (Input.GetKeyDown (KeyCode.Escape)) 
 {
         // TODO....
 }

or

 if (Input.GetKey("escape"))
 {        
          Application.Quit();
 }

or

 public void Update ()  
 {
         if (Application.platform == RuntimePlatform.Android) 
         {
             if (Input.GetKeyUp(KeyCode.Escape)) 
            {
                //quit application on return button
                Application.Quit();
                return;
            }
         }
     }

or something similar... This is non-working code! What you need to do to earn !? PLEASE HELP ME!