How to Register custom fields for the REST API

register-meta-for-rest.php

<?php
	$args = array(
		'type'              => 'string',
		'description'       => 'The International Standard Book Number',
		'single'            => true,
		'sanitize_callback' => 'sanitize_isbn',
		'auth_callback'     => 'is_allowed_to_change_isbn',
		'show_in_rest'      => true,
	);

	register_meta( 'post', 'isbn', $args );