Passing URL param to hidden field

Is it possible to pass a URL parameter to a hidden field? I can’t get it to work.

Custom plugin adds the parameter ‘special’

add_filter('query_vars', 'parameter_queryvars' );

function parameter_queryvars( $qvars ) {
    $qvars[] = 'special';
    return $qvars;
}

Metabox setup tries to assign it to a $parentref variable:

add_filter( 'rwmb_meta_boxes', 'mylink_register_meta_boxes' );
function mylink_register_meta_boxes( $meta_boxes ) {
	global $wp_query;
	if (isset($wp_query->query_vars['special']))
	{
		$parentref = $wp_query->query_vars['special'];
	}

and later

array(
                'id'   => $prefix . 'hidden',
                'type' => 'hidden',
                'std'  => $parentref,
            ),

the add url for the metabox post looks like
http://mydomain.com/wp-admin/post-new.php?post_type=mylink&special=278

but there’s no value for the hidden field in the database. (all the other metabox fields work perfectly)

https://wordpress.org/plugins/meta-box/