Seems easy enough. If you have an Android device including Amazon Fire Tablets, download the Hay Day app for free and get $10 Amazon appstore credit. I have no idea what the game is about, but if the credit is indeed only for apps then I can buy an educational app for my preschooler. (We like the Endless series by Originator, and Endless Alphabet is free with Amazon Underground.)
For a limited time, customers who download Hay Day from Amazon Appstore between August 1, 2016 and August 31, 2016 will receive a $10 credit toward purchase of digital content in Amazon Appstore. Only customers who have not previously downloaded Hay Day from Amazon Appstore are eligible for this offer.
Update: This is showing up for me as a $10 Amazon credit towards anything shipped and sold by Amazon. Just add any items over $10 that is shipped and sold by Amazon to your cart, and you should see this. I just added some enzymatic dental chews for my dogs for $12.08 and saw this:
As of 8/3, they added the following sentence to the terms and conditions: “Please note, it may take up to 48 hours for you to receive an e-mail confirming that the $10 Amazon credit has been applied to your account.” As of this day, the credit also appears to be limited to App Store purchases as well.