Adding Custom Endpoints to The Events Calendar

functions.php

// this filter adds the custom field values to the json output for The Events Calendar
// https://theeventscalendar.com/support/forums/topic/custom-field-output-for-json-feed/

add_filter( 'tribe_rest_event_data', 'omfg_add_event_customfields' );
   
function omfg_add_event_customfields($data) {
  $event_id = $data['id'];
  $some_data = 'some data';
  $data = array_merge( $data, 
        // This is for a single value
        array('band_id' => $some_data,
    ));
    
  return $data;
}