In Unity, I want to get mouse input in the background (just click) when the window is inactive.

It works normally if it is active, but not if it is inactive.
The build did not change.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class ClickMouse: MonoBehaviour
{
    public void OnGUI ()
    {
        Event e = Event.current;
        if (e.button == 0&&e.isMouse&&e.type == EventType.MouseDown)
        {
            this.gameObject.GetComponent<SpriteRenderer>(). color = new Color (1, 1, 1, 0);
        }
    }
}

There is an article about using Event to receive input, but I tried it, but it does not work.
https://qiita.com/kaikiofkaiki/items/f705716f9e45c11b5288

You can get keyboard input using "UnityRawInput" which is also introduced.

There is almost no other information and I'm stuck.

Unity2018.4.12f1