All coupon codes can be applied at the end of the free trial, when you choose a plan and add your payment details.
Look for the "Got a coupon or referral code?" link, just below where you add your credit card details.
Click "Got a coupon or referral code?" and a text field will open where you can add your code and validate it.
When your coupon is validated, you will see a green "Valid" message.
If you have questions about a specific coupon, please contact customer support and we'll get you answers.