You have to apply using your official email address only. Promo code is sent on official email addresses only. However, there is no restriction of using promo code and you can use that even for an account registered on authorSTREAM with non official email address(Yahoo,Gmail etc).
In case you don’t have any official email address then you can send support@authorstream.com any information which can prove that you are a employee of an educational institute. It may be university website link mentioning your profile or some other proof.