Call SubscriberService.CreateSubscriberWithDoubleOptIn
Create a subscriber and add a subscription to a specific mailing list in a double opt-in mode. Subscriber is created as pending and an email is sent to them with a link they can use to confirm their subscription. When they click that link, the pending flag is removed and any confirmation email and/or SMS is sent to them based on recipient settings and mailing list settings. If this method is called for a specific email before the subscriber confirms their subscription, another email with the confirmation link is sent. However if it's called after the subscriber confirms their subscription, no email is sent and the subscriber data is updated. If a given email is on opt-out list/all subscriber and subscription will not be created and confirmation link will not be sent.