How to test Subscriptions properly?

[sqrter]

Hi again,

Prerequisites: I want to use the subscriptions='Y' feature but have no idea how to test it properly. I couldn't find a way to test it through the merchant portal so let me ask a few questions here.

1. There's a subscription_callback_url parameter in the order creation request. If I skip it, will the default callback_url be used for recurring subscription callbacks?
2. How does the subscription callback payload look like? Is it different in any way from the one that is sent when the first payment is performed? Could you please provide a JSON example of the response payload for subscription callback?
3. Is there any way to understand that the callback was called by a subscription? There's some parent_order_id parameter passed to the callback. It's always empty, but maybe when a callback is triggered by subscription it will include the original order_id?