Paste code

<?php
$user_ref = 'payload-' . strval(time());
$app_id = 12345;
$page_id = 6789; // with domain whitelisted
?>

<html>
<body>
<script>
  window.fbAsyncInit = function() {
    FB.init({
      appId            : '<?=$app_id?>',
      autoLogAppEvents : true,
      xfbml            : true,
      version          : 'v4.0'
    });
    FB.Event.subscribe('send_to_messenger', function(e) {
      console.log('Got event: ' + e.event + ' with ref:' + e.ref);
      if(e.event == 'opt_in'){
        alert('Got opt-in for ' + e.ref);
      }
    });
  };
</script>
<script async defer src="https://connect.facebook.net/en_US/sdk.js"></script>


<h3>Send to Messenger Plugin Test</h3>
<div style="height:100px">
<div class="fb-send-to-messenger" 
  messenger_app_id="<?=$app_id?>" 
  page_id="<?=$page_id?>" 
  data-ref="<?=$user_ref?>" 
  color="blue" 
  size="standard">
</div>
</div>

<p><span>user_ref:&nbsp;</span><?=$user_ref?></p>


<script>
  FB.Event.subscribe('send_to_messenger', function(e) {
    // callback for events triggered by the plugin
    console.dir(e);
      });

      </script>

</body>
</html>