Hey, this is the workaround that I use

1. Go to website editor and enter the edit mode
2. Click theme tab and scroll down
3. Under the Website Settings, click on "</body>" button beside the code injection field
4. Add the following code

<script>
var links = document.querySelectorAll('a.nav-link.dropdown-toggle[href="#"] > span');
var MenuA = Array.from(links).find(span => span.textContent.trim() === 'Menu A');
if (MenuA) {
	​var link = MenuA.parentElement;
	​link.addEventListener('click', function(e) {
	​	​e.preventDefault();
	​	​window.location.href = '/menu-a';
	​});
}
</script>

It is kind of hardcoded, but if you want another alternative, please refer to this link: https://www.odoo.com/en_US/forum/help-1/website-navbar-menu-links-with-sub-menu-200257

That's a good question. 

I cannot believe this functionality is not included by default or nobody has posted a solution for such a simple question :(