If you came to Brilliant through a sponsorship link, the discount will be automatically applied to your account. If you go to the premium page, you should see the discounted price next to the crossed-out original price, as shown below:
Coupons do have a limited lifetime, however, and typically expire within 48 hours of being activated. If your coupon has gone missing, this is likely the reason.
Still have questions? Reach us at support@brilliant.org.