How can I create a menu item corresponding to a link to another (external) site? In my menu I can create an internal link corresponding to a path like → '/path/to/node'.

But when trying to create a link to an external website (like → http://www.external.com), the link actually becomes → https://www.mywebsite.com/http://www.external.com

So basically, external links are not recognized as external. But instead they get added to the base URL.

How can I make this to work?
You are not allowed to add external link to menu items as this link confirms it too.