Show Ads when Unity game starts

SimpleAds.cs

using UnityEngine;
using System.Collections;
using UnityEngine.Advertisements;

public class SimpleAds : MonoBehaviour 
{
    
    //Change to your gameID code in
    //unityads.unity3d.com
    string gameID = "33675";
    
    void Start () 
    {
        Advertisement.Initialize (gameID, true);

        StartCoroutine (ShowAdWhenReady());
    }

    IEnumerator ShowAdWhenReady()
    {
        while (!Advertisement.isReady())
            yield return null;

        Advertisement.Show ();
    }
}