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Webhooks in PlayStream (Analytics->Webhooks) automatically send the complete body of the PlayStream event to your endpoint, including all parameters. You can see the parameters for events in the PlayStream docs, here: https://api.playfab.com/playstream/events. If you need custom data, you can use WritePlayerEvent to create the event, and the "Custom event name" selection in the webhook configuration, with your event's actual name, to generate a webhook to your server.