I am trying to set up a recurring payment with PayPal Subscription REST API. I'm following the Basic Integration in PayPal Developer Portal, using as suggested the Smart Payment Button. Besides, I'm using a WebHook to "capture" events related to subscription. As the tutoria suggest, I created a Product and a Plan. Now, I rendered the Smart Payment Button in a page (using the example), and when clicked it triggers the PayPal authorization flow. When the user complete the subscription process, the BILLING.SUBSCRIPTION.CREATED event is triggered. Later, also the PAYMENT.SALE.COMPLETED and BILLING.SUBSCRIPTION.ACTIVATED are triggered, and I receive data in my WebHook. Now, I have troubles in identifying which user activated the subscription. My idea is to pass a custom variable (with something allows me to identify the user) and retrive it later when the WebHook post me data, but I can't undertand how to do it. Anyway, is this the best way to do it? Or there is another solution?