I dont think you can now. That was the bad offer. This one has been arround for years.
You can also buy a $50 gift card and get $10 credit back. Here is the link with the code to apply to your account. You may also just be able to enter "GCPRIME16 ".
EDIT: You can get another $10 promotion credit if you sign up for a free Audible trial through Amazon. You can cancel immediately. The Audible's "expires at 11:59 p.m. (PT) 90 days from date the promotional credit is received."