How can I track purchases from external landing pages?

Hi,

While users navigate through the site and make purchases, we also have external landing pages that use /cart/add?id=productid permalinks. These enable the landing pages to have an add to cart button (without it needing to be a buy button or sales channel complications) that simply loads the product in cart.

What I want to do is track and report on purchases derived through these permalinks but unable to find a way to do so.

Would love some help please.