How can I use raycasthit and collider raycast to hit only if the mouse position is over a specific object ?

 private void Update()
     {
         if (Input.GetMouseButtonDown(1))
         {
             RaycastHit hit;
             Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
             if (terrainCollider.Raycast(ray, out hit, Mathf.Infinity))
             {
                 GameObject cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
                 cube.transform.position = hit.point;
                 cube.transform.localScale = new Vector3(50, 50, 50);
             }
         }
     }

This is working fine when I click with the mouse on the terrain it will spawn a cube on that position I clicked with the mouse.

But now I added a plane a small size plane above the terrain the plane is about 0.5 higher from the terrain.

I want that I click with the mouse it will spawn the cube/s on the terrain but only if the mouse is inside the plane area. Not anywhere on the terrain like it is now but only on the plane area.

I mean that only when I click with the mouse somewhere on the plane area than spawn cube on the terrain.

On the top of the script I reference to my plane :

 public CustomPlane plane;

And this is the CustomPlane script :

 using UnityEngine;
 
 [ExecuteAlways]
 [RequireComponent(typeof(MeshFilter), typeof(MeshRenderer))]
 public class CustomPlane : MonoBehaviour {
 
     public float width = 1;
     public float length = 1;
 
     private float oldWidth;
     private float oldHeight;
 
     public void Start()
     {
         oldWidth = width;
         oldHeight = length;
 
         Create();
     }
 
     private void Update()
     {
         if(oldWidth != width || oldHeight != length)
         {
             Create();
 
             oldWidth = width;
             oldHeight = length;
         }
     }
 
     private void Create()
     {
         MeshFilter meshFilter = gameObject.GetComponent<MeshFilter>();
 
         Mesh mesh = new Mesh();
 
         Vector3[] vertices = new Vector3[4]
         {
             new Vector3(0, 0, 0),
             new Vector3(width, 0, 0),
             new Vector3(0, length, 0),
             new Vector3(width, length, 0)
         };
         mesh.vertices = vertices;
 
         int[] tris = new int[6]
         {
             // lower left triangle
             0, 2, 1,
             // upper right triangle
             2, 3, 1
         };
         mesh.triangles = tris;
 
         Vector3[] normals = new Vector3[4]
         {
             -Vector3.forward,
             -Vector3.forward,
             -Vector3.forward,
             -Vector3.forward
         };
         mesh.normals = normals;
 
         Vector2[] uv = new Vector2[4]
         {
             new Vector2(0, 0),
             new Vector2(1, 0),
             new Vector2(0, 1),
             new Vector2(1, 1)
         };
         mesh.uv = uv;
 
         meshFilter.mesh = mesh;
     }
 }