curl --request PUT \
  --url https://cronho.st/api/v1/schedules/{id}/notifications \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '
{
  "notifyOn": "failure",
  "channelIds": [
    "channel_123"
  ]
}
'

{
  "data": {
    "scheduleId": "schedule_123",
    "notifyOn": "failure",
    "channels": [
      {
        "channelId": "channel_123",
        "label": "Ops alerts",
        "verified": true
      }
    ],
    "warning": null
  },
  "success": true
}