Promo code for new buyers - where to put it in?

[readingswithkat]

I received this (not much time left) but as far as I an tell I can’t find anyplace to input the promo code. I’m trying to check out and can’t find a place to use this… anyone else successfully use a promo code?

Get 20% off your first order!
Use promocode: STARTDOINGTODAY
Applies only to desktop purchases.
Valid until 25.01.2018 23:59 GMT