Steps for Creating Direct Link to Checkout:
1) Log Into Your Winible Account
2) Navigate to Your Store
3) In the top right corner, click on Public View
4) Select the Subscription Plan Which You Want to Direct Traffic to
5) Copy the URL out of the Browser. This is your direct link to checkout
Steps for Auto Applying Promo Codes to that Checkout Link:
1) Edit Your Subscription Plan and Make Promo Codes
2) Take Your Checkout Link From Above
3) Adding &c=PROMOCODE to the End of Any Checkout Link to Auto Apply Said Promo Code