Replace {INSERTPAGEIDHERE} with your FB page id.

Facebook Chat SDK Script

<script>
  var chatbox = document.getElementById('fb-customer-chat');
  chatbox.setAttribute("page_id", "{INSERTPAGEIDHERE}");
  chatbox.setAttribute("attribution", "biz_inbox");
</script>

<!-- Your SDK code -->
<script>
  window.fbAsyncInit = function() {
	FB.init({
	  xfbml            : true,
	  version          : 'v15.0'
	});
  };

  (function(d, s, id) {
	var js, fjs = d.getElementsByTagName(s)[0];
	if (d.getElementById(id)) return;
	js = d.createElement(s); js.id = id;
	js.src = 'https://connect.facebook.net/en_GB/sdk/xfbml.customerchat.js';
	fjs.parentNode.insertBefore(js, fjs);
  }(document, 'script', 'facebook-jssdk'));
</script>