For the prerequisite steps, such as how to install Unity Advertisement package and integrated into project, please have a look at this:
Interstitial adv is a full screen ad, it can be displayed between loading different game levels, game finished and etc. In this case, we want to show the Interstitial ad when the level starts, so first to create a empty game object named InterstitialAdManager and them attach the following script to it:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Advertisements;
public class InterstitialAdManager : MonoBehaviour {
#if UNITY_IOS
private string gameId = "3685328";
#elif UNITY_ANDROID
private string gameId = "3685329";
#endif
public string placementId = "video";
public bool testMode = true;
void Start() {
// Initialize the Ads service:
Advertisement.Initialize(gameId, testMode);
// Show an ad:
StartCoroutine(ShowInterstitialAdWhenReady());
}
IEnumerator ShowInterstitialAdWhenReady () {
while (!Advertisement.IsReady (placementId)) {
yield return new WaitForSeconds (0.5f);
}
Advertisement.Show();
}
}