Enter it on the review page and the discount shows before you pay.
If the campsite has given you a promo code, enter it on the review page before paying. The discount is applied to your total straight away, so you can see exactly what you'll pay.
Codes are set up by each campsite and usually have a validity window. If a code isn't accepted, check it hasn't expired and that you've typed it exactly, then ask the campsite.
Email support@pillowcase.io with your campsite name or booking reference and we'll pick it up from there.