add_action('rest_api_init', function () {
    // ‘post’ 文章类型，也可以是其他文章类型，‘my-my_post_meta’ 是前端提交过来的数据
    register_rest_field('post', 'my_post_meta', array(
        // 把自定义字段数据附加到 Rest API 文章接口返回的 Json 数据里
        'get_callback' => function ($object) {
            $my_post_meta = get_post_meta($object['id'], 'my_post_meta', true);
            return $my_post_meta;
        },
        // 保存数据的回调，获取前端提交过来的数据，然后保存
        'update_callback' => function ($value, $post_object) {
            $post_id = $post_object->ID;
            add_post_meta($post_Id, 'my_post_meta', $value, true) or
                    update_post_meta($post_id, 'my_post_meta', $value);
            return true;
        }
    ));
});

register_meta(
  'post',
  'oss_url',
  array(
    'single' => true,
    'type' => 'string',
    'show_in_rest' => true
  )
);

###
POST https://dev.rangtuo.com/wp-json/wp/v2/audios/162
content-type: application/json; charset=utf-8
Authorization: Bearer eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpYXQiOjE2OTkwMzAyNDQsImV4cCI6MTY5OTA5MDI0NCwibmJmIjoxNjk5MDMwMjQ0LCJ1c2VyX2lkIjoiMSJ9.uesgf5_cYKGXOyGmEOghsj8TiTX2Kh-npJhdR2R0rbI

{
    "title":"一个测试用的5",
    "content":"这是内容",
    "my_post_meta":"这里是自定义字段的值"
}