1) Sign into your account here.
2) If you have multiple subscriptions, click on the "Meal Planner" in the upper left-hand corner and make sure you select the correct subscription.
3) Click on the link that says "Subscription Settings" on the left underneath the list of your recent weekly orders.
4) Click on the "Add" button next to where it says "Promo Code."
5) Enter the promo code you want to apply to your subscription in the resulting pop-up window and click "Apply."
If the promo code is valid, it will be entered onto your order and you will see it in your subscription settings!
Please note, however, that just because a promo code is valid and entered on the subscription, that does not mean it is valid for your next charge. Some promo codes are only valid for new customers or only valid for certain plans (for instance, 25% off the 6-meal plan). If your current subscription does not adhere to the rules for that promo code, the code would not apply to your next charge.