Paste the code to you Shift4Shop store’s HTML field.
Next, go to your Shift4Shop store and find the HTML field. It is most likely in the form of an HTML/Code widget or module which you can find on Shift4Shop’s dashboard. Add it to your page, and paste the code you’ve copied before to the HTML field. Save and update, and that’s it, you should have Common Ninja’s Countdown app on your Shift4Shop store.