This script lets you choose the destination point on the NavMesh by clicking the mouse on the object’s surface. The position of the click is determined by a raycast, rather like pointing a laser beam at the object to see where it hits (see the page Rays from the Camera for a full description of this technique). Since the GetComponent function is fairly slow to execute, the script stores its result in a variable during the Start function rather than call it repeatedly in Update.
    // MoveToClickPoint.cs
    using UnityEngine;
    
    public class MoveToClickPoint : MonoBehaviour {
        NavMeshAgent agent;
        
        void Start() {
            agent = GetComponent<NavMeshAgent>();
        }
        
        void Update() {
            if (Input.GetMouseButtonDown(0)) {
                RaycastHit hit;
                
                if (Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), out hit, 100)) {
                    agent.destination = hit.point;
                }
            }
        }
    }
    //MoveToClickPoint.js
    var agent: NavMeshAgent;
    
    function Start() {
        agent = GetComponent.<NavMeshAgent>();
    }
    function Update() {
        if (Input.GetMouseButtonDown(0)) {
            var hit: RaycastHit;
        
            if (Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), hit, 100)) {
                agent.destination = hit.point;
            }
        }
    }