curl --request DELETE \
  --url https://trade.predexon.com/api/accounts/{accountId}/orders \
  --header 'x-api-key: <api-key>'

{
  "cancelled": 123,
  "_meta": {
    "venues": [
      {
        "venue": "<string>",
        "status": "ok",
        "cancelled": 123
      }
    ]
  }
}