Genesys Cloud Webhook for Slack on SLA Breach - Payload Issue

How do I set up a Genesys Cloud webhook to send Slack notifications when a queue breaches its SLA? I’ve tried using the /api/v2/routing/webhooks endpoint. The webhook creates fine, but the payload sent to Slack is empty. Here is the JSON I’m sending:

{
 "eventDefinition": {
 "eventType": "routing.queue.sla.breach"
 }
}

The status code is 201, but nothing hits Slack. Am I missing a mapping step?