Embedding a video source from YouTube is as simple as adding the YouTube URL to the source tag and specifying the type to be video/youtube:

<div class="jw-settings" id="my_simple_settings" data-skin="/haiku/haiku.xml">
     <video id="my_simple_video" controls poster="/posters/big-buck-bunny-preview.jpg" preload="none" >
          <source src="http://www.youtube.com/watch?v=ShbC5yVqOdI" type="video/youtube" />
     <div class="jw-modes">
          <div class="jw-mode" data-type="html5"></div>
          <div class="jw-mode" data-type="flash" data-src="/jwplayer/v5/player.swf"></div>

This results in the following YouTube video being wrapped in the JW Player: