This is an example data format for a new custom event that is received and a POST request as JSON sent to your Webhook URL.

{
  "trigger": "new",
  "lead_id": 153928,
  "user_id": "51497-af17340d-62b8-3044-423f-3dc754e621c2",
  "lead_type": "Custom Event",
  "lead_status": "Unique",
  "date_created": "2016-02-01T14:09:01Z",
  "quotable": "Not Set",
  "quote_value": null,
  "sales_value": 50,
  "profile": "My Profile",
  "profile_id": 51497,
  "account_id": 27313,
  "lead_url": "https://www.whatconverts.com/contact",
  "landing_url": "https://www.whatconverts.com/",
  "event_category": "Lead",
  "event_action": "New Signup",
  "event_label": "Click",
  "additional_fields": {
    "full_name": "John Doe",
    "email_address": "hello@whatconverts.com"
  },
  "lead_source": "google",
  "lead_medium": "cpc",
  "lead_campaign": "lead generation",
  "lead_content": "new ad",
  "lead_keyword": "generating leads",
  "ip_address": "107.210.21.188",
  "gclid": "CLibmtmqpNICFcSfGwodQbUAvg",
  "msclkid": "25d83debf85f146b8f1d66a754c6a56c",
  "salesforce_user_id": "15228103840",
  "duplicate": false,
  "spam": false
}

For additional information on setting up webhooks see Webhooks.