nee help with Instantiating prefab at mouse position with GUI button

hi, I finally got my code working that lets me so far lengthen a piece of road prefab thats already in the scene to any length i want and also rotate it by clicking on it and dragging it. My camera is a overhead camera that i can rotate and move around. What I need help with is i need to instantiate the road prefab by clicking on a GUI button then click and drag it to the length i want and place it at that length on the terrain (kind of like cities xl or simcity).

Problem is that none of the methods i tried seem to work and i also cant find anything on google that would help me. here is the code.

 using UnityEngine;
 using System.Collections;
 
 public class Roadplacement : MonoBehaviour 
 {
     public GameObject Road;
     public GameObject RoadPrefab;
 
     private Ray ray;
 
     private float distObjRay;
     private float RoadLength;
     private float RoadScale_z;
     private float hitdist = 0.0f;
     private float speed = 30.0f;
 
     private Vector3 objectPos;
     private Vector3 Raypoint;
     private Vector3 Raypointxz;
 
 
     // Update is called once per frame
     void Update () 
     {
         
         if(Input.GetMouseButton(0))
         {
             // Initial scale and position vector of road
             Road.transform.localScale = new Vector3(Road.transform.localScale.x,Road.transform.localScale.y,0f);
 
             //Ray is being cast at mouse Position
             Plane roadPlane = new Plane(Vector3.up, transform.position);
             ray = Camera.main.ScreenPointToRay (Input.mousePosition);
 
             // World Coordinates of Ray
             Vector3 Raypoint = ray.GetPoint(hitdist);
             Vector3 Raypointxz = new Vector3(Raypoint.x,0,Raypoint.z);
             Vector3 objectPos = new Vector3(Road.transform.position.x,0,Road.transform.position.z);
 
             //Distance of object to Ray
             float distObjRay = Vector3.Distance(Raypointxz,objectPos);
 
             //Rotation of Road object towards mouse position
             if (roadPlane.Raycast (ray, out hitdist))
             {
                 Vector3 targetPoint = ray.GetPoint(hitdist);
                 Quaternion targetRotation = Quaternion.LookRotation(targetPoint - transform.position);
                 transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, speed * Time.deltaTime);
 
             }
 
             // New Scale based on the distance between mouse position and the object
             RoadScale_z = distObjRay *0.125f;
 
             // Applies the new z scale
             Road.transform.localScale = new Vector3(Road.transform.localScale.x,Road.transform.localScale.y,-RoadScale_z);
 
             // Tiles the road texture according to new scale of the prefab
             RoadPrefab.renderer.material.mainTextureScale = new Vector2(1,Road.transform.localScale.z *2);
 
         }
     }
 }

I think it needs to instantiated on input.getmousebuttonup after i dragged the road prefab with input.getmousebutton . I also think i need to save the inital mouse position on the ground which would be the starting point for the road. I dont know how to implement all that to get it to work. The code i have so far works fine with one road prefab in the seen. Maybe someone can help understand how instantiating actually works. Thanks.