This is a plugin I made last year but due to several circumstances I just published it around a month ago.

It let's you create a custom URL (link) that will let you automatically apply a coupon and optionally a product when visiting the URL. It's a WooCommerce plugin and it's meant to be 100% free (no upgrade or pro version).

I'd love to read what you think and which ways I can improve it (event the marketing/wording).