Create Webhook failed

I want to create a webhook that listens to all the events associated with my organization. Below is my request object.

var requestBody = new WebhookSubscriptionRequest
{
    url = "https://00f9-2001-ee1-df19-e6e0-a953-99e6-1ea6-928c.ngrok-free.app/Calendly/Callback",
    events = new string[] { "invitee.created", "invitee.canceled" },
    scope = "organization",
    organization = orgUrl,
    signing_key = "ABC",
};

Then I got this error:
 

But in the document, it looks like the group and user parameters are optional. Am I missing something?