Make an object a trigger?

I am making a racing game. When the player passes though the finish line banner (Has a box collider with isTrigger checked) I want my victory panel to appear (it would become active)
I don’t know how to use triggers though, and any advice would help!

My code:

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

public class FinishRace : MonoBehaviour
    public GameObject finishScreen;
    // Start is called before the first frame update
    void Start()
    { finishScreen.SetActive(false);
        //so basically
         * When the flag is touched, stop the time
         * bring up the game over screen and show the time that the player got
         * pause everything???
         * have a button that brings over the next track

    // Update is called once per frame
    void Update()
    public class Finish : MonoBehaviour
        private void OnTriggerEnter(Collider other)

Thank you in advance!

I would personally add a tag to the trigger box, something like “Finish”.
For the car script add

public FinishRace finishrace; //Create reference to other script

void OnCollisionEnter(Collision other) 
        if(other.gameObject.tag == "Finish")
            //Do other stuff here to stop time etc