I also noticed that I got an email from Arby's titled "Happy Sandwichversary! Or Arbysversary! It's our anniversary!". In the email there was the same coupon. So if you are already subscribed you might already have a similar email you can look for.
The coupon I got said choose from: Reuben, Loaded Italian, Smokehouse Brisket, or Any Gyro.
Sort of ran across the sign up page for the same offer on Google.