Intercept JSON Booking Success message

<?php

/**
This snippet will allow users to modify successful booking message
*/
function em_mod_success_message($result,$EM_Booking){
 $html = $EM_Booking->get_event()->output('<script>alert( " TEST: #_EVENTNAME " );</script>');
 $result['message'] = $EM_Booking->output(get_option('dbem_booking_feedback') . $html );
 return $result;
}
add_filter('em_action_booking_add','em_mod_success_message',100,2);
//or
//add_filter('em_booking_save','em_mod_success_message',100,2);