Find psid after clicking fb-send-to-messenger button

Question

window.fbAsyncInit = function() {

FB.init({

  appId            : 'app_id',

  autoLogAppEvents : true,

  xfbml            : true,

  version          : 'v3.2'

});     

FB.Event.subscribe('send_to_messenger', function(response) {   
 if ( response.event == 'clicked' ) {

 };

});

};

(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_US/sdk.js";

 // js.src = "https://connect.facebook.net/en_US/sdk/debug.js";

 fjs.parentNode.insertBefore(js, fjs);

}(document, 'script', 'facebook-jssdk'));

I am trying to execute the facebook send to messenger api by using facebook javascript sdk . I already have an app which have the webhook events subscribed .So i want to find the psid after clicking the fb-send-to-messenger .