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).
More on this, I I'd like to get feedback on whether it's intuitive or not (I went with a builder which is what I personally find intuitive but I'd love to hear what more seasoned (and unbiased) people think).