Creates a subscription that bills a customer on a recurring cadence, in the live or test mode of the API key used. Each item defines its price through price_data, referencing an existing product (product) or defining one inline (product_data). All items must share the same currency and the same recurring interval and interval_count, and a subscription can have up to 10 items. Fintoc sets the billing_cycle_anchor to the creation time, and the collection_method is always send_invoice. Fintoc generates and finalizes an initial invoice for the first billing period immediately, and marks zero-amount invoices as paid automatically.