问如何将posts中定义的自定义字段添加到wordpress中的rest API响应中

EN

{
    "_links": {
        "about": [
            {
                "href": "http://example.com/wp-json/wp/v2/types/post"
            }
        ],
        "author": [
            {
                "embeddable": true,
                "href": "http://example.com/wp-json/wp/v2/users/1"
            }
        ],
        "collection": [
            {
                "href": "http://example.com/wp-json/wp/v2/posts"
            }
        ],
        "curies": [
            {
                "href": "https://api.w.org/{rel}",
                "name": "wp",
                "templated": true
            }
        ],
        "replies": [
            {
                "embeddable": true,
                "href": "http://example.com/wp-json/wp/v2/comments?post=21"
            }
        ],
        "self": [
            {
                "href": "http://example.com/wp-json/wp/v2/posts/21"
            }
        ],
        "version-history": [
            {
                "href": "http://example.com/wp-json/wp/v2/posts/21/revisions"
            }
        ],
        "wp:attachment": [
            {
                "href": "http://example.com/wp-json/wp/v2/media?parent=21"
            }
        ],
        "wp:featuredmedia": [
            {
                "embeddable": true,
                "href": "http://example.com/wp-json/wp/v2/media/23"
            }
        ],
        "wp:term": [
            {
                "embeddable": true,
                "href": "http://example.com/wp-json/wp/v2/categories?post=21",
                "taxonomy": "category"
            },
            {
                "embeddable": true,
                "href": "http://example.com/wp-json/wp/v2/tags?post=21",
                "taxonomy": "post_tag"
            }
        ]
    },
    "author": 1,
    "categories": [
        5,
        4
    ],
    "comment_status": "open",
    "content": {
        "protected": false,
        "rendered": ""
    },
    "date": "2017-05-14T15:25:33",
    "date_gmt": "2017-05-14T15:25:33",
    "excerpt": {
        "protected": false,
        "rendered": ""
    },
    "featured_media": 23,
    "format": "standard",
    "guid": {
        "rendered": "http://example.com/?p=21"
    },
    "id": 21,
    "link": "http://example.com/2017/05/14/post/",
    "meta": [],
    "modified": "2017-05-15T18:17:34",
    "modified_gmt": "2017-05-15T18:17:34",
    "ping_status": "open",
    "slug": "",
    "sticky": false,
    "tags": [],
    "template": "",
    "title": {
        "rendered": ""
    },
    "type": "post"
}