WP REST API — Adding Custom Fields to Custom Route

Hi all,

we know that adding a custom field to a native route of the WP REST API is pretty easy, and it looks something like this:

function user_favorites_count() {
    // Return the user's favorite count
}

add_action( 'rest_api_init', function () {

    register_rest_field( 'user', 'favorites_count', array(
        'get_callback' => 'user_favorites_count',
        'schema' => null
    ));

});

However, this does not seem to work for custom routes: for example, if I created a custom route named customer and wanted to use it instead of user as the register_rest_field, the favorites_count field does not get registered. So how do you add a custom field to a custom route?