How to create a link that adds an item to the cart?

I noticed that the direct links used to add the items to a cart no longer work.

I verified what the other authors are doing and it seems that in all the items that use this method this feature doesn’t work.

As an example, this is the link used in the item description of the most popular WordPress plugin on CodeCanyon. This link doesn’t work.

I’m asking because I’m interested in applying this method for our items.