PayPal Smart Subscribe server side
I'm not aware of a demo sample, but the createSubscription portion can be be done via this API call: https://developer.paypal.com/docs/api/subscriptions/v1/#subscriptions_create
For subscriptions, by default the subscription will be activated automatically on approval, before onApprove is called.
You can set the application_context -> user_action to something other than SUBSCRIBE_NOW if you want to show a review step and activate with an API call.