Add meta data to WP-API custom post type without authorization (example here is for Tribe Events Calendar)

functions.php

<?php
add_filter( 'json_prepare_post', function ($data, $post, $context) {
$data['myextradata'] = array(
    'start_date' => get_post_meta( $post['ID'], '_EventStartDate', true ),
    'end_date' => get_post_meta( $post['ID'], '_EventEndDate', true ),
);
return $data;
}, 10, 3 );