For those having trouble here’s a hint. Look harder at the email you get confirming your order with amazon. The 16 digit code is there. Simply go to Xbox and redeem the code. Even easier, use the Xbox app on your phone.
It did not work for me initially (said code had been redeemed). I tried via the xbox itself and via the link amazon provided. In the end I managed to redeem the code through the main microsoft website. After this I was able to buy the game I wanted via the website and then download it on the xbox.
Received a code to input on Microsoft Xbox site (can probably do this from the console too) and entered the code to redeem. It credited my Xbox account with the amount pretty much immediately. Ideal for "gift carding" if you don't know what the other person likes to play.