WP REST API on Existing Custom Post Types

rest-api-existing-cpt.php

<?php
/**
 * Add REST API to an existing custom post type
 * Remember to add existing custom fields
 */

function add_cpts_to_api( $args, $post_type ) {
  if ( 'post_name' === $post_type ) {
    $args['show_in_rest'] = true;
  }
  return $args;
}

add_filter( 'register_post_type_args', 'add_cpts_to_api', 10, 2 );