Triggered when a user registers for a calendar event

{
  "version": "0.0.1",
  "type": "calendar_event.user_registered",
  "triggered_at": "<string>",
  "action": "calendar_event.user_registered",
  "custom_metadata": "<string>",
  "event": {
    "id": "<string>",
    "title": "<string>",
    "slug": "<string>"
  },
  "source": {
    "slack_workspace_id": "<string>",
    "slack_channel_id": "<string>"
  },
  "user": {
    "profile_id": "<string>",
    "created_at": "<string>",
    "preferred_name": "<string>",
    "first_name": "<string>",
    "last_name": "<string>",
    "email": "<string>",
    "slack_user_id": "<string>",
    "slack_is_admin": true,
    "slack_is_bot": true,
    "slack_first_name": "<string>",
    "slack_last_name": "<string>",
    "slack_image_72": "<string>",
    "slack_image_original": "<string>",
    "profile_fields": {}
  }
}