I have managed to create an anchor from a custom menu link...
@viky081, you appear to have misunderstood the question.
Back to the point, I have managed to use the custom url for making anchors to a post further down on the same page.
All you need to do is add the post id to the custom menu item.
Where it asks for the url, just type in the post ID --> #post-6329
And give it a label.
When the link is clicked, it takes one to that post on the same page.
You don't need to use the post ID though. If you create an anchor on the page wherever you want, eg a post about how to make pickled onions:
<a href="#pickled-onions">Pickled Onions</a>
Then just insert #pickled-onions into the custom url field for the menu on that page. When you click the custom menu link it will jump to the anchor.
However, what I'd really like to do is add a scroll event, so that when clicked the page scrolls to the anchor, instead of jumping straight there. I can do this easily with ordinary links, but having problems getting it to work with WP menu links as anchor links.