is just for watching 4 hours straight and no more, right?
You also need to link a blizzard account (I believe with a copy of overwatch) to your twitch. Make sure your watching one of the drops enabled streamers aswell.
Baring in mind, don't quote me on this I got 1st wave invite to the beta instead of through drops so I couldn't tell you if it works or not. Also I don't think blizzard expected just how missive the return of Overwatch would be and the servers are currently being mega nuked with the number of people