That’s not a special link — just kmart.com.
The only thing you need to see the discount is an account. After you’re logged in, if you add anything to your cart, you should see that you have “FREECASH” available up to $20, which you can apply at the checkout page to bring your total to $0.
After I did my first order, it looks like I earned another $6 in “FREECASH” so I placed a second order of a smaller item.
Do in-store pickup to save on shipping costs. Technically this can work once per account. Not sure if these orders will be honored.