Coupon to register an account

[Planit]

Hi there,

I'm looking for a mod that allows me to give out coupons|code|phrase which allows people to register on the forum. Without a coupon they can't register.

I have included an example image.

Please note I am new to vbulletin so if there is an inbuilt feature that can do this please excuse my ignorance.

Edit: I forgot to mention that I could add a new user profile field, make it mandatory at registration and set the regular expression to match the coupon code but I'm not sure if there is a more elegant way to do so.