According to their press release, anybody with a free Healthy Rewards account will be rewarded with a free Watermelon Smoothie!
From June 1 to June 3, guests can visit this page and enter their favorite summer activity to get a free 12oz. watermelon smoothie of their choice, redeemable through June 7 with Smoothie King's Healthy Rewards app!
Update: Here's the direct link to the promo coupon!
The only problem is I don't see this offered anywhere on that page! I've tweeted at Smoothie King asking what's up, so hopefully we'll get an answer soon. In the meantime, check out the link above to see if there's any mention about a free smoothie.
Smoothie King also gives out birthday freebies, which you'll qualify for by making an account.