curl --request POST \
  --url https://api.unibee.dev/merchant/invoice/external_gateway_invoice/mark_refunded \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "externalTransactionId": "<string>",
  "invoiceId": "<string>",
  "signature": "<string>",
  "timestamp": 123,
  "gatewayId": 123
}
'

{
  "code": 123,
  "data": {
    "invoiceId": "<string>",
    "refundId": "<string>",
    "status": "<string>"
  },
  "merchantId": 123,
  "message": "<string>",
  "redirect": "<string>",
  "requestId": "<string>"
}