Several of the shops that use our app have requested this feature. Once your customer goes through a discount link, then adds items to the cart, our app will attempt to apply the discount and show them their savings. All you need to do is use the standard Shopify discount link: "https://yourshop.com/discount/THE_CODE"
You can test this out for 14 days risk-free: https://apps.shopify.com/discount-on-cart-pro
Hope this helps!