How to add schema.org SiteNavigationElement and property URL to navbar?

A little searching got me the nav_menu_link_attributes filter:

function wpse183311_filter( $atts, $item, $args ) {
    $atts['itemtype'] = 'http://schema.org/SiteNavigationElement';
    return $atts;
}
add_filter('nav_menu_link_attributes', 'wpse183311_filter', 3, 10);

Add however many attributes you need to the $atts array as necessary