add_action( 'pre_get_posts', 'orderby_customfield' );

function orderby_customfield( $query )
{
    if (is_admin()){
        return;
    }

  if ( $query->is_search ) {
            $query->set('orderby', 'kurumsal');
            $query->set('posts_per_page', '20');
            $query->set('order', 'ASC' );
        }
    return $query;