unity - How to make projectiles shoot at mouse position?

I'm looking to make it so my fired projectile is shot at the position of the mouse so aiming is an option ie:up down ect..

I have the script working to where it spawns the cloned object and launches it but it only shoots straight from my players hand before disappearing. how can I adjust my command so the object is shot in the direction the mouse is pointing?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ProjectileShooter : MonoBehaviour {

  [SerializeField]
  Transform attachmentPoint;
  GameObject prefab;


  void Start () {

    prefab = Resources.Load ("projectile") as GameObject;   
  }


  // Update is called once per frame
  void Update () {
    if (Input.GetMouseButtonDown (0)) {
      GameObject projectile = Instantiate (prefab) as GameObject;
      projectile.transform.position = attachmentPoint.position + Camera.main.transform.forward * 2;
      Rigidbody rb = projectile.GetComponent ();
      rb.velocity = Camera.main.transform.forward * 40;
      Destroy (projectile, 2.5f);
    }
  }

}