Click Button and Follow Mouse

So I have this code, if I start the game the code that I put in the button makes the button follow the mouse. What I want is if I click that certain button that is when that button follows the cursor because I would implement it to my click and drop function. I have tried putting the code on the OnClick() function of the button but what it does is whenever I click on the button it follows the point where my mouse was clicked. So here is the code.

     using UnityEngine;
     using System.Collections;
     
     public class FollowMouse : MonoBehaviour {
         
         // Use this for initialization
         public void Start () 
         {
         
         }
         // Update is called once per frame
         public void Update()
         {
             float distance = 1.0f;
             bool useInitialCameraDistance = true;
             float actualDistance;
                 if (useInitialCameraDistance) {
                     Vector3 toObjectVector = transform.position - Camera.main.transform.position;
                     Vector3 linearDistanceVector = Vector3.Project (toObjectVector, Camera.main.transform.forward);
                     actualDistance = linearDistanceVector.magnitude;
                 } else {
                     actualDistance = distance;
                 }
                 Vector3 mousePosition = Input.mousePosition;
                 mousePosition.z = actualDistance;
                 transform.position = Camera.main.ScreenToWorldPoint (mousePosition);
         }
     }

Thank you in advance.