Hurry over to Bing.com and search the word 'Amazon', then scroll down to the bottom and look for the 'A Free Gift for You' offer (looks like this). Create an account or log in and select the 'Your sign-up bonus' offer (Looks like this). Then claim your gift card! Enter it on Amazon.com when you receive it via email.
This offer does not always show up with a new account, so be aware. :)