It's the link that's created by Microsoft for their promo codes. Linking to the app's page itself would have been a problem because it's not free. It's only free with the promo code.

In another comment on this thread I added a link to the app's page, and the promo code as a self standing code so you can use it by yourself.

The login page is AFAIK a page to log in to your Microsoft account to use the code. But as mentioned above - see another comment of mine with the details of how to do it manually.