Application singleton use in Android

I have a facebook initialize sdk call and I need it to initialize the moment application is launched:

I want to use my Application class to do that. for example:

public class App extends Application {

@Override
public void onCreate() {
    super.onCreate();
    FacebookSdk.sdkInitialize(getApplicationContext());
}
}

I have the main activity with the facebook login button:

public class MainActivity extends AppCompatActivity {
@BindView(R.id.login_button)
LoginButton loginButton;

private CallbackManager callbackManager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ButterKnife.bind(this);

    callbackManager = CallbackManager.Factory.create();
    loginButton.registerCallback(callbackManager, new FacebookCallback<LoginResult>() {
        @Override
        public void onSuccess(LoginResult loginResult) {

        }

        @Override
        public void onCancel() {

        }

        @Override
        public void onError(FacebookException error) {

        }
    });
}
}

How do I call my Application singleton? How do I make its onCreate() work?

To make your App class singleton follow the Singleton design pattern:

public class App
  {
    // Create the instance
    private static App instance;
    public static App getInstance()
    {
         if (instance== null) {
                synchronized(App.class) {
                        if (instance == null)
                                instance = new App();
                        }
                }
                // Return the instance
                return instance;
    }

    private App()
    {
        // Constructor hidden because this is a singleton
    }

    public void initFBSdk()
    {
        FacebookSdk.sdkInitialize(getApplicationContext());
    }
}

Then to use it in any other class:

App.getInstance().initFBSdk();

If this is what you are asking for..