How to store a gameobject in a variable

I have a script attached to a cube in my project, I want to activate and deactivate the cube when I press return and shift key respectively, but I can't do that in the script attached to the object as once the object is disabled, the script can't be accessed anymore, so I created another cube and attached a script to it, I put in this code:

 using UnityEngine;
 using System.Collections;
 
 public class Enable_Disable_Script : MonoBehaviour {
     private cubeScript myScript; 
 
     // Use this for initialization
     void Start () 
     {
         myScript = new cubeScript();
     }
     
     // Update is called once per frame
     void Update () 
     {
         if (Input.GetKeyDown (KeyCode.Return)) {
             myScript.Cube.SetActive (false);
         } 
         else if (Input.GetKeyUp (KeyCode.Return)) 
         {
             myScript.Cube.SetActive(true);
         }
     }
 }

NOTE: 'cubeScript' is the script I attached to the first cube and 'Cube' is the name of the variable I want to store my cube gameobject in.

My question is - How can I store the cube(gameobject) that I want to activate/deactivate in the variable 'Cube' by using some code in the first script?

Please help, I am a beginner in unity and I don't know how to do this, I have tried each and everything I knew and nothing is working, I have spent hours trying to figure it out, but it seems I am getting it wrong somehow, any help will be appreciated!