The Links tab is one of the product tabs which appears with product details in each of these screens: the Catalogues screen, the Search screen or when you view Suppliers > Individual Prices
Open the 
Catalogues window and locate a product using the Search options to access these options on the Links tab
A product may have default web links, and you can add your own.
Click 
Add to enter a title and website address.
Click 
Edit to amend a link you previously added (you cannot edit links that were included in the catalogue data)