Unfortunately it is not possible to add a voucher after placing an order.
You can enter the voucher code in your order overview. You will get to the order overview after you have clicked on "Checkout" in your shopping cart. There is a grey font with "coupon code available" directly above the green field "Buy now". If you click on it, you will find the field to enter the voucher. If the code has been accepted, you will see this in the order overview.