require(['jquery'],
    $(document).ready(function() {
        $("#nav li a").attr("target", "_blank");
    });
);

<add id="Vendor_Module::id"
             title="Menu title"
             module="Vendor_Module"
             sortOrder="60"
             parent="Vendor_Module::parent"
             action="your_action"
             resource="Vendor_Module::resource"
             target="_blank" />