add following code in your theme’s (child theme’s if any) functions.php
add_action( 'fep_action_message_after_send', function( $message_id, $message, $inserted_message ){
$post = array(
'ID' => $message_id,
'post_title' => $inserted_message->post_title . ' #' . $message_id,
);
wp_update_post( $post );
}, 10, 3);