Although the download won't cost you a dime, you'll have to provide a valid email address. Doing so will sign you up for Bing's newsletter, so you might want to use a junk account, just make sure you can log in. Once you supply your email, Bing will send a code and instructions to download a song, which is as simple as finding a tune on Bing Music, choosing the option to buy it via Amazon, and checking out with your code.
It's worth mentioning that the email also comes with a confirmation link for the newsletter subscription, and we're not sure if Microsoft registers your subscription without clicking through. Either way, you don't have to click that verification link to download a song. The promotion is limited to US residents over the age of 13 and it will end New Year's Eve at midnight. You can read the full list of terms and conditions here.