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. This option will be on the left side of the screen on desktop and in the top right corner on mobile.
To apply a code to an existing subscription — for instance, when upgrading from an individual to a community subscription — sign in, then 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.