Introduction

Since it took me a day to figure it out and several hours of reading comments and suggestions which didn't quite work, let me help you by saving you time.

Background

I was looking for a way to control Pause event from "Game" window.

To be more specific, I wanted to catch the following event:

I read so many answers such as:

    void OnApplicationPause( bool pauseStatus )
    {
        isPaused = pauseStatus;
    }

or:

    void OnGUI( )
    {
        if( isPaused )
            
    }

which was not what I was looking for.

I continued searching and trying more things like: OnDisable, OnApplicationPause, OnApplicationFocus, OnUpdate, OnLateUpdate, OnFixedUpdate....

I've seen many things like this:

	void Update () 
     {
		if(Input.GetKeyDown(KeyCode.P)) //or Input.GetKey(KeyCode.Pause)
		{
			if(Time.timeScale == 1 && playerController.alive == true)
			{
				Time.timeScale = 0;
				showPaused();
			} else if (Time.timeScale == 0 && playerController.alive == true){
				Time.timeScale = 1;
				hidePaused();
			}
		}

At a certain point, I even thought about ways to detect my mouse position and set "hit" event when pressing my mouse, then trying to guess whether I hit the pause button.

Luckily for me, I'm too lazy to do that, so I continued reading and found these two links:

Success!

Using the Code

A breakthrough!
I continued reading more about the play mode and understood that I needed to catch those events.
https://docs.unity3d.com/Manual/GameView.html

using UnityEditor;

        void OnEnable()
        {
        #if UNITY_EDITOR
            EditorApplication.playmodeStateChanged += StateChange;
        #endif
        }

        #if UNITY_EDITOR
        void StateChange()
        {
            if (EditorApplication.isPlayingOrWillChangePlaymode && EditorApplication.isPlaying) 
            {
              Debug.Log("GOT IT");
            }
        }
        #endif
        //-------------------------------------------------------------------------------------

and this is the right event for those issues.

Good luck!

Unity3D - How to Detect Pause Event from "Game" Window