Right now when you sign up for the Habit Charclub, you will receive a free Charburger offer by email.
Just go here and fill out the short signup form (and it's free, of course!), then your coupon will be emailed to you within a couple hours. This is for a limited time, so I would sign up now!
NOTE: I am just signing up now myself, so I cannot confirm when and if you will receive the coupon–it's worth a try!