Shhhhh, @Ann86. Saying you got superhost for nothing is going to stir up a hornet's nest! Seriously, though, there is no indication on a person's Airbnb profile that they have a credit. People complain about that all the time. You have to save the email. And there is no danger to you, that I can see, if you use the code they give. The email is not telling you to go to a certain sketchy site, right? With your next booking over $100, when you are signed in legitimately to Airbnb, just enter that code at the first opportunity (near the beginning of the process) and it should work.
Something you can check, though. When July 14 comes, just check Standards in your Stats to see whether you are indeed a superhost.