rest api – register_rest_field only for specific page

I am using register_rest_field to send an additional field to the page endpoint but it is sending it to every page. I only want it on a specific page but don’t know how to pass the page id to the function.

add_action('rest_api_init', 'register_test' );

function register_test(){
    register_rest_field( array('page', 21),
            'get_callback'    => 'get_id',
            'update_callback' => null,
            'schema'          => null,

function get_id( $request ) {

$featured_posts = get_field('recommended');
foreach( $featured_posts as &$featured_post) {
    $featured_post->tester = get_field( 'recommended_link', $featured_post->ID );
return $featured_posts;