Add a reminder at 60 minutes before event's datetime start. Event Espresso 4.

ical_reminder.php

<?php // open PHP tag must be omitted if adding the following code to an existing PHP file that already has an opening PHP tag

add_filter(
  'FHEE__EED_Ical__download_ics_file_ics_data',
  function(
    $ics_data, $datetime
  ) {
    $ics_data['BEGIN'] = "VALARM";
    $ics_data['TRIGGER'] = "-PT60M";
    $ics_data['ACTION'] = "DISPLAY\r\nDESCRIPTION:Reminder";
    $ics_data['END'] = "VALARM";
    return $ics_data;
  },
  10,
  2
);