Example for jetpack contact_form_message filter hook

change_succesful_jetpack_contact_form_submission_email_message.php

<?php
function dd_change_successful_contact_form_submission_email_message($text){

$new_text = '<p>Text to add after email message. <a href="https://example.com/" target="_blank">Visit Example.com!</a></p>';
$text = $text . $new_text;
  return $text;
}

add_filter ('contact_form_message', 'dd_change_successful_contact_form_submission_email_message');