post https://api.fastspring.com/subscriptions//convert
Creates a session that allows a buyer to reactivate a trial subscription and convert it to a paid subscription.
Preconditions:
- The subscription is a trial without any payment method added.
- The subscription was deactivated because the trial period ended without a payment method being added.
- The subscription conversion period is still active.
- Conversion is enabled for the subscription.
- The reactivation request is made within the allowed conversion period: Deactivation date + conversion days <= API call date.