WooCommerce Coupon Links 
Automatically apply WooCommerce coupon codes to the cart via a URL.
Contributors: Luke McDonald, Brady Vercher
Requires: 4.7
Tested up to: 4.9
License: GPL-2.0+
Usage
First, you will need to set up a coupon in WooCommerce. The coupon can then be applied by adding a coupon_code query argument to the cart URL.
https://audiotheme.com/cart/?coupon_code=highfive
If the cart is empty, the coupon will still be applied and visible once the user adds a product.
Probably more useful than just applying a coupon code to the URL, is appending the coupon code argument to the end of a product's "Add to Cart" URL.
https://audiotheme.com/view/promenade/?add-to-cart=2804&coupon_code=highfive
In the above example, a product with the id of 2804 will be added to the cart with the highfive coupon applied.
Installation
- Download the latest release from GitHub.
- Go to the Plugins โ Add New screen in your WordPress admin panel and click the Upload button at the top next to the "Add Plugins" title.
- Upload the zipped archive.
- Click the Activate Plugin link after installation completes.