Promotion codes are redeemed on the subscription checkout page.
To apply one to a new subscription, go through the steps to create a new subscription. On the checkout page where you enter your payment details, select "Add promotion code" and apply the code there.
To apply one to an existing subscription — for instance, when upgrading from an individual to a community subscription — go to the "your settings" page and select "Manage subscription." Adjust the duration and/or the number of people (quantity) on your subscription, then select "Add promotion code" and apply the code before confirming changes.
Note: Some codes are only valid toward the monthly plan. If you apply a code to the annual plan and do not see the expected discount, you may need to apply the code to the monthly plan instead.