Example of how to add a custom message shortcode to the EE_Datetime_Shortcodes library.

ee_register_new_custom_messages_shortcodes.php

<?php
/* This filter allows you to add custom shortcodes to the message system
 * $shortcodes is an array of the available shortcodes for the current library
 * $lib is the current shortcode library
 */
function ee_register_new_custom_messages_shortcodes( $shortcodes, EE_Shortcodes $lib ) {
  
	//Add a shortcode to be used with the EE Datetimes within messages
	if ( $lib instanceof EE_Datetime_Shortcodes ) {
		//Add your shortcode to the add as the key, the value should be a description of the shortcode.
		$shortcodes['[DATETIME_ID]'] = _('The ID of the current Datetime');
	}
	//Return the shortcodes.
	return $shortcodes;
}
add_filter( 'FHEE__EE_Shortcodes__shortcodes', 'ee_register_new_custom_messages_shortcodes', 10, 2 );