I've got a script working nicely where a user visiting a WordPress site can fill out their name and email address, check the opt-in box, and sign up to a list on NationBuilder. But I can't seem to find any information on how to have NationBuilder send a verification email or double opt-in message to the new user to confirm that the email is valid and they do really truly want to be on this list. How would we go about this using the API?