The signup method, how to get payment information from customer, one of the following: source, email, link. The source method takes a reference to an existing customer payment method (e.g. card ca_..) or a card token for ct_.... The method email will send an email to customer requesting payment information with a link to a hosted page. The method link does not do anything but the hosted page link in the subscription object can be provided to the customer as a link to enter payment information.