Subscribes a customer to a plan. The behavior depends on your billing mode and plan timing:

Tanso mode (no Stripe): Returns the new subscription with its initial invoice. For in-advance plans, the subscription stays inactive until the invoice is marked paid. For in-arrears plans, the subscription activates immediately.

Stripe Integration + in-advance plans: Returns a checkoutUrl instead of a subscription. Redirect your customer to this URL to collect payment via Stripe Checkout. Once payment completes, Tanso receives a webhook and automatically creates the subscription, invoice, entitlements, and credits. Free plans ($0) skip checkout and activate immediately.

Stripe Integration + in-arrears plans: The subscription activates immediately and is synced to Stripe. Stripe invoices at the end of the billing period.

The customer is identified by your customerReferenceId. Retrieve available plan IDs from the Plans endpoint.