Selective fetch meta data via rest api

Hi,

I'm probably doing something wrong, but I'm trying to fetch a restricted set of meta fields from a post using the REST API.

/wp-json/wp/v2/book/16

Returns a field containing all data

    "meta_box": {
        "url": "https://google.com",
        "text": "asdfasdf"
    },

But when I try to select a single value I get 0 values.

/wp-json/wp/v2/book/16?_fields=meta_box

returns

"meta_box": [],

What I am trying to get is a query that returns only specific Meta Box values. ie just meta_box.url

eg /wp-json/wp/v2/book/16?_fields=title.rendered,excerpt.rendered,meta_box.url

to get something like

    "title": {
        "rendered": "The Harder We Fall"
    },
    "excerpt": {
        "rendered": "<p>Insomnia. It’s part of the penance I pay for my greatest mistake. But when an ill-timed doze behind the wheel of my car nearly introduces me to a pole, I know something has to change.</p>\n"
    },
    "meta_box": [
       "url": "https://google.com"
],

My googling has returned nothing useful for me. Your assistance would be appreciated.