how to implement in wordpress a signup flow that requires a secret activation code

I'm trying to implement a signup flow in wordpress with the additional requirement that a user, to sign up, must use a secret activation code. Such code is valid only once, it's consumed after signup.

Many such codes are available (manually imported in DB is fine).

The use case is to let sign up only users who bought a (paper) book containing the secret activation code. In each book, the code will be different.

Before writing the whole thing manually, is there a plugin with a similar functionality? If not, what is the suggested implementation strategy?

via Chebli Mohamed