Add a custom meta field to wordpress API response

functions.php

function my_rest_prepare_post( $data, $post, $request ) {
  $_data = $data->data;
  $_data['URL'] = get_post_meta( $post->ID, 'URL', true );
  $data->data = $_data;
  return $data;
}
add_filter( 'rest_prepare_post', 'my_rest_prepare_post', 10, 3 );