Use this request with a BillingTemplate having is_subscription == true. Two scenarios are possible:
• If subscription_charge_period_end == true and/or subscription_trial_periods > 0 (first billing should happen after 1 or more billing periods, not today), the request will create a BillingTemplateClient and start trial/schedule billing for it (as required by subscription settings). Successful response will be of form {billing_template_client: <BillingTemplateClient object created>, purchase: null}: no Purchase is created, BillingTemplateClient.status is active immediately.
• If subscription_charge_period_end == false and subscription_trial_periods == 0 (first billing should occur today), the request will create a BillingTemplateClient with status == pending and create a Purchase. When such a Purchase is paid, the respective BillingTemplateClient will have its' subscription activated (starting from the day of payment), with its status changing to active. Successful response will be of form {billing_template_client: <BillingTemplateClient object created>, purchase: <Purchase object created>}: you should redirect your client to purchase.checkout_url for him to pay immediately (as you do with POST /purchases/).