var tag = document.createElement('script');
    tag.src = "https://www.youtube.com/player_api";
    var firstScriptTag = document.getElementsByTagName('script')[0];
    firstScriptTag.parentNode.insertBefore(tag, firstScriptTag);
    var player;
    function onYouTubePlayerAPIReady() {
        player = new YT.Player('ytplayer', {
            height: '360',
            width: '640',
            videoId: 'ZF_1vqGZgw0'
        });
    }

    onYouTubePlayerAPIReady();

<div class="slide" id="ytplayer"></div>

Uncaught ReferenceError: YT is not defined