Add key `thumbnail_url` in WordPress REST API

functions.php

<?php
add_action('rest_api_init', 'register_rest_images' );
function register_rest_images(){
	register_rest_field( array('post'),
		'thumbnail_url',
		array(
			'get_callback'    => 'get_rest_featured_image',
			'update_callback' => null,
			'schema'          => null,
		)
	);
}

function get_rest_featured_image( $object, $field_name, $request ) {
	if( $object['featured_media'] ){
		$img = wp_get_attachment_image_src( $object['featured_media'], 'app-thumb' );
		return $img[0];
	}

	return null;
}