Add format to docvalues in elasticsearch js client request

Hi,
Using the Elasticsearch client js, I am able to add the doc values to the request like so:

callWithRequest(_req, 'search', {
    index: indexPatternTitle,
    scroll: '1m',
    body: body,
    docvalue_fields:'timestamp'
});

But I want to be able to specify the format.
In the console I can add the field "format": "MM/d/YYYY h:mm:ss a"

And the result will be formatted according to the format I specified.

But I am not able to pass the format using the js client.
I tried passing an object with like:

{
    "field": "timestamp",
   "format": "MM/d/YYYY h:mm:ss a"
}

But it doesn't work.

Any Idea on How to do that? or could it even be done using the js client?

Thanks.