VIBSHAKE
Pretty self explanatory - just use coupon code online with the burger and shake in your order, or present email to staff when ordering. Not sure how strict they are about it, but to be on the safe side I'd suggest using the "Forward to a friend" link in the email I've linked to avoid trouble.
Please note: email suggests that soy shakes are also free, but they don't trigger an order credit when added to an order.
