Pre-Roll-Video

<!-- 1. Carrega o IMA SDK (antes do seu código) -->
<script src="//imasdk.googleapis.com/js/sdkloader/ima3.js"></script>

<!-- 2. O vídeo e, por cima dele, a caixa do anúncio -->
<div style="position:relative;width:640px;height:360px">
  <video id="conteudo" width="640" height="360" playsinline controls>
    <source src="meu-video.mp4" type="video/mp4">
  </video>
  <div id="adContainer" style="position:absolute;top:0;left:0"></div>
</div>
<button id="playButton">Assistir</button>

<script>
  var video = document.getElementById('conteudo');
  var adDisplayContainer = new google.ima.AdDisplayContainer(
      document.getElementById('adContainer'), video);
  var adsLoader = new google.ima.AdsLoader(adDisplayContainer);
  var adsManager;

  adsLoader.addEventListener(
      google.ima.AdsManagerLoadedEvent.Type.ADS_MANAGER_LOADED,
      function (e) {
        adsManager = e.getAdsManager(video);
        adsManager.addEventListener(
            google.ima.AdEvent.Type.CONTENT_PAUSE_REQUESTED,
            function () { video.pause(); });
        adsManager.addEventListener(
            google.ima.AdEvent.Type.CONTENT_RESUME_REQUESTED,
            function () { video.play(); });
        adsManager.addEventListener(
            google.ima.AdErrorEvent.Type.AD_ERROR, onAdError);
      });
  adsLoader.addEventListener(
      google.ima.AdErrorEvent.Type.AD_ERROR, onAdError);

  function onAdError(e) {
    console.log(e.getError());
    if (adsManager) { adsManager.destroy(); }
    video.play(); // sem anúncio, o vídeo continua
  }

  // 3. Pedido do anúncio (IDs de exemplo)
  var adsRequest = new google.ima.AdsRequest();
  adsRequest.adTagUrl = 'https://pubads.g.doubleclick.net/gampad/ads?' +
      'iu=/1234567/exemplo&sz=640x480&env=vp&gdfp_req=1&output=vast' +
      '&unviewed_position_start=1&vpa=click&vpmute=0&plcmt=1' +
      '&description_url=https%3A%2F%2Fwww.exemplo.com%2Fvideo&correlator=';
  adsRequest.linearAdSlotWidth = 640;
  adsRequest.linearAdSlotHeight = 400;
  adsLoader.requestAds(adsRequest);

  // 4. Só começa quando a pessoa clica em Assistir
  document.getElementById('playButton').addEventListener('click', function () {
    adDisplayContainer.initialize();
    try {
      adsManager.init(640, 360, google.ima.ViewMode.NORMAL);
      adsManager.start();
    } catch (err) {
      video.play();
    }
  });
</script>