On the final page of checkout, underneath your payment information will be text that reads “Add Promo Code or Discount." Underneath that text will be a box to enter your promo/voucher code.
Also, if you have previous store credits, Groupon, Living Social or other third party vouchers, they will appear beneath the "Add Promo Code or Discount" text.
Please Note: Some promo codes can only be used for certain Bouq collections. Be sure to check the link on the promotional email you received to ensure you are shopping the applicable collection.