Automatically Add Coupon To Cart via Query String URL Parameter
Many if not most promotions these days (and 90% of mine for the past ten years) are handled via newsletter and social media links that direct a user to a landing page such as a product or category page on a website store. As such it is always easier for the customer to purchase directly from a link that automatically adds a coupon discount code to the cart (and obviously validates it first before doing so) by grabbing the code from the url query string and not relying or expecting the customer to also note down the coupon code and manually apply it to the cart, whether or not there are items in the cart.
The idea of the checkout process is to remove obstacles and speed up the flow to purchase and such implementation would do this. Store admin should be able to generate a promotion url that redirects a potential customer to a category page or a product page or auto-add a product to the cart and at the same time auto-add a specified coupon to the cart too.
Richard Abbott commented
This is also a required feature if you wish to offer a discount to selected customers and hide the actual coupon code so people can't abuse a promotion by posting the coupon code via social media. If it is hidden in a url and automatically added to the cart then the customer cannot see the coupon code but the discount is then applied when the appropriate product is added to the cart.