Still just add a link in nav manager, and associate it with your product.
When you export the product, it will export the link in the xml.
Seems that my mind has messed at all. Ok, I clicked at Navigation Manager, I selected Add Link from Forum selector and then I added a link. I was unable to choose product as vBulletin was preselected (see attached screenshot). But the links appears ok.
The novice question now. What product do I need to download to copy the code? I downloaded the style but there is nothing there. I seen in the Products list but only CMS and Blog exist.