<div id="ytplayer"></div>
<script type="text/javascript">
// Load the IFrame Player API code asynchronously.
  var tag = document.createElement('script');
  tag.src = "https://www.youtube.com/player_api";
  var firstScriptTag = document.getElementsByTagName('script')[0];
  firstScriptTag.parentNode.insertBefore(tag, firstScriptTag);

  // Replace the 'ytplayer' element with an <iframe> and
  // YouTube player after the API code downloads.
  var player;
  function onYouTubePlayerAPIReady() {

  player = new YT.Player('ytplayer', {
      height: '390',
      width: '640',
      videoId: 'M7lc1UVf-VE', 
    // place your youtube videoID here
    events: {
    'onReady': onPlayerReady 
    // 'onReady' will execute a function when the player is loaded
    }
    })

  }
  function onPlayerReady(){player.playVideo()} 
  // function that plays video
// ]]></script>
<button onclick="onPlayerReady()">play</button>
<!-- calls function that affects player -->