Call SubscriberService.CreateSubscriber
Create a subscriber on account and add subscription to a list. If the email address already exists on the account, only a subscription to the list will be created (and if UpdateIfExisting is "true", the subscriber data will be updated). Be aware that if you are using CreateSubscriber endpoint in a subscription form you should also check and delete the subscriber from OptOutAll list before. NB! In case of bulk imports/updates - you should use one of the methods in the Import service. (Direct)