Edit in JSFiddle

<!DOCTYPE html>
<html>
<head>
  <!-- This page is an example of prebid and JWPlayer integration with
       a JWPlayer playAd call once the player is ready for playback -->
  <meta charset="utf-8" />
  <title>Prebid.js Self-Hosted JW Player Example</title>

  <!-- prebid.js -->
  <script src="https://acdn.adnxs.com/prebid/not-for-prod/1/prebid.js" async=true></script>

  <!-- JWPlayer -->
    <script type="text/javascript" src="http://ssl.p.jwpcdn.com/player/v/7.2.4/jwplayer.js"></script>
    <script type="text/javascript">
       jwplayer.key="YEUwUA9wRFeDcydr"; //Test Key - replace this with your own valid JWPlayer license key
    </script>
  <script>

    var pbjs = pbjs || {};
    pbjs.que = pbjs.que || [];

    // define invokeVideoPlayer in advance in case we get the bids back from prebid before the entire page loads
    var tempTag = false;
    var invokeVideoPlayer = function(url){
      tempTag = url;
    }

    /*
    Prebid Video adUnit
    */

    var videoAdUnit = {
      code: 'video1',
      mediaTypes: {
        video: {
          context: "instream",
          playerSize: [640, 480]
        }
      },
      bids: [
        {
          bidder: 'appnexusAst',
          params: {
            placementId: '9333431', // Add your own placement id here
            video: {
              skipppable: true,
              playback_method: ['auto_play_sound_off']
            }
          }
        }
      ]
    };

    var logBids = function(bids){
      console.log('MESSAGE: got bids back: ');
      console.log(bids);
      if (!bids.video1) {
        console.log('MESSAGE: no video demand');
      }
      else {
        console.log('MESSAGE: we got video demand!');
      }

      // Log the VAST URL, if there is one
      try {
        url = bids.video1.bids[0].vastUrl;
        console.log('MESSAGE: VAST URL: ');
        console.log('MESSAGE: ' + url);
      } catch (e) {
        console.log("MESSAGE: There was an error logging the vast url: " + e);
      } // ignore if there is no vastUrl
    }

    pbjs.que.push(function(){
      pbjs.addAdUnits(videoAdUnit); // add your ad units to the bid request

      pbjs.setConfig({ 
        cache: {
           url: "https://prebid.adnxs.com/pbc/v1/cache"
         }
      });

      pbjs.requestBids({
          bidsBackHandler: function(bids) {
              var videoUrl = pbjs.adServers.dfp.buildVideoUrl({
                  adUnit: videoAdUnit,
                  params: {
                      iu: '/19968336/prebid_cache_video_adunit',
                      cust_params: {
                        section: "blog",
                        anotherKey: "anotherValue"
                      },
                      output: "vast"
                  }
              });
              invokeVideoPlayer(videoUrl);
          }
      });
    });

    pbjs.bidderSettings =
    {
        standard: {
            adserverTargeting: [
                {
                    key: "hb_bidder",
                    val: function (bidResponse) {
                        return bidResponse.bidderCode;
                    }
                }, {
                    key: "hb_adid",
                    val: function (bidResponse) {
                        return bidResponse.adId;
                    }
                }, {
                    key: "hb_pb",
                    val: function (bidResponse) {
                        return "10.00";
                    }
                }, {
                    key: "hb_size",
                    val: function (bidResponse) {
                        return bidResponse.size;

                    }
                }
            ]
        }
    };

  </script>
</head>

<body>

  <h1>Prebid Video - JWPlayer</h1>
  <br>
  <br>
  <div id="playerContainerJW" style='width:640px; height:480px; border:1px solid black;'>
  </div>

  <script>
    var jwPlayerInstance = jwplayer("playerContainerJW");

    invokeVideoPlayer = function(url) {
        jwPlayerInstance.setup({
            "file": "http://video-demo.appnexus.com/pbjs/JWPlayerDemo/AppNexus_Summit_Video_HighRes.mp4",
            "width": 640,
            "height": 480,
            "autostart": true,
            "mute": true,
            "advertising": {
                client: "vast",
            }
        });

        jwPlayerInstance.on('beforePlay', function(){
            jwPlayerInstance.playAd(url);
        })
    }

    if (tempTag) {
        invokeVideoPlayer(tempTag);
        tempTag = false;
    }
  </script>
</body>
</html>