Additional attribute for menu link
Try
function HOOK_link_alter(&amp;$variables) or function HOOK_preprocess_menu(&amp;$variables)
Or just override the menu template (menu--main.html.twig) and replace line
{{ link(item.title, item.url, {'class': link_classes}) }}
with
{{ link(item.title, item.url, item.attributes.addClass(link_classes).setAttribute('itemprop', 'url') }}