scripting problems please help

hi, can i set the zombie variable from this script to be set as a object from within the scene in code, i tried a lot of codes no one realy worked using UnityEngine; using System.Collections;

 public class SpawnScript : MonoBehaviour {
     public int zombieamount = 4;
     public GameObject zombie;
 
     // Use this for initialization
     void Start () {
         Instantiate(zombie, transform.position, transform.rotation);
     }
     
     // Update is called once per frame
     void Update () {
         if (zombieamount <= 0)
         {
             Instantiate(zombie, transform.position, transform.rotation);
             zombieamount = 4;
         }
     }
     public void Zombiecount()
     {
         zombieamount -= 1;
     }
 }