How do I attach an object to my pointer when i click a gui button?

Hello! I'm fairly new to coding and I don't really know how to do this, I'm trying to create a tower defense game an i'm in the process of making a build functionality in it but this is my first encounter to using GUI buttons, this is what I got so far

     public List<GameObject> Build;
     
     void OnGUI()
     {
             if (GUI.Button(new Rect(15, 15, 100, 50), "Red"))
             {
                 Debug.Log("clicked");
                 GameObject g =  (GameObject)Instantiate(Build[i]);
             }
     }
 .....................
 

Here's is my code for the object

     using UnityEngine;
     
     using System.Collections;
     
     public class dragobject : MonoBehaviour
     {
         private Vector3 screenPoint;
         private Vector3 offset;
         
         void OnMouseDown() 
         { 
             screenPoint = Camera.main.WorldToScreenPoint(gameObject.transform.position);
             
             offset = gameObject.transform.position - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
             
             Cursor.visible = false;
         }
     
     void OnMouseDrag() 
     { 
         Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
         
         Vector3 curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
         
         transform.position = curPosition;
         
     }
     
     void OnMouseUp()
     {
         Cursor.visible = true;
     }
 }