How to drag an empty into a prefab

hey guys, I'm new to unity and iI'm trying to make my own game. After + 50hours coding on it, I'm stuck. In fact i created a prefab called "mob" and i want it to move to an empty which is randomly moving in an area all the 2 seconds. Anyways, i created my prefab and now i can't drag my empty in my prefab but I can if my IA isn't a prefab and i spawn it my by myself.

here is my IA code and some screenshot to explain you in detail:

http://www.noelshack.com/2018-43-7-1540700627-drag.png

http://www.noelshack.com/2018-43-7-1540700627-prefab.png

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ShootIA : MonoBehaviour {
     public float speed;
     public float stoppingDistance;
     public float retreatDistance;
     public float DetectionDistance;
 
     private float waitTime;
     public float startWaitTime;
 
     public Transform moveSpot;
     public float minX;
     public float maxX;
     public float minY;
     public float maxY;
 
     public GameObject projectile;
     private Transform player;
     private bool DepAutorise = true;
 
     // Use this for initialization
     void Start () {
         player = GameObject.FindGameObjectWithTag("Player").transform;
         waitTime = startWaitTime;
         moveSpot.position = new Vector2(Random.Range(minX, maxX), Random.Range(minY, maxY));
     }
     
     // Update is called once per frame
     void Update () {
         
         //print(transform.position);
 
         if (transform.position.x > minX && transform.position.x < maxX && transform.position.y > minY && transform.position.y < maxY && DepAutorise )
         {
             if ((Vector2.Distance(transform.position, player.position) > stoppingDistance) && (Vector2.Distance(transform.position, player.position) < DetectionDistance))
             {
                 transform.position = Vector2.MoveTowards(transform.position, player.position, speed * Time.deltaTime);
             }
             else if (Vector2.Distance(transform.position, player.position) < stoppingDistance && Vector2.Distance(transform.position, player.position) > retreatDistance)
             {
                 transform.position = this.transform.position;
             }
             else if (Vector2.Distance(transform.position, player.position) < retreatDistance)
             {
                 transform.position = Vector2.MoveTowards(transform.position, player.position, -speed * Time.deltaTime);
             }
             if (waitTime <= 0 && (Vector2.Distance(transform.position, player.position) < stoppingDistance))
             {
                 Instantiate(projectile, transform.position, Quaternion.identity);
                 waitTime = startWaitTime;
             }
             else
             {
                 waitTime -= Time.deltaTime;
             }
         }
         else { 
         transform.position = Vector2.MoveTowards(transform.position, moveSpot.position, speed * Time.deltaTime);
         waitTime = startWaitTime;
             DepAutorise = false;
             if (Vector2.Distance(transform.position, moveSpot.position) < 0.02f) 
             {
                 DepAutorise = true;
                     }
         }
 
 
 
 
     }
 }
 

my spawner :

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Spawner : MonoBehaviour {
     public float rotationSpeed;
 
 
     private float waitTime;
     public float startWaitTime;
 
     public GameObject mob;
 
 
     // Use this for initialization
     void Start () {
         waitTime = startWaitTime;
     }
 
     // Update is called once per frame
     void Update()
     {
         transform.Rotate(Vector3.forward * rotationSpeed * Time.deltaTime);
 
         if (waitTime <= 0)
         {
             Instantiate(mob, transform.position, Quaternion.identity);
             waitTime = startWaitTime;
         }
 
         if (waitTime >= 0)
         {
 
             waitTime -= Time.deltaTime;
         }
     }
 }

Maybe you have a solution or a way to not use an empty, but I'm seriously stuck. Thanks in advance, Naexys.