If you have a discount coupon code, then you can apply it during the purchase of a new subscription or the purchase of new seats.
That is, from your Dashboard when you create a new organization, when you add new seats to your organization, or when you click buy while on Trial. You can also purchase directly from the website's pricing page.
When you click on the "Add discount code" link you get the field to enter the code and click "Apply". The amount indicated will be adjusted accordingly.