post
https://sandbox.api.tansoflow.com/api/v1/client/subscriptions
Subscribes a customer to a plan and immediately returns the new subscription with its initial invoice. For usage-based and hybrid plans, the invoice reflects the base charge (if any) — usage charges accumulate through the Events endpoint and appear on the next billing cycle invoice.
The customer is identified by your externalClientCustomerId. Retrieve available plan IDs from the Plans endpoint.