Signature Validation Helper

curl --location --request POST 'https://subscription.be.pkbsvc.com/webhook/onerway/callback' \
--header 'Content-Type: application/json' \
--data-raw '// ---------------------- Instructions ----------------------
// 1. Configure your credentials in the environment variables:
//    - merchantNo: Your merchant account number
//    - secret: Your secret key
//    - appId: Your application identifier
//
// 2. Replace the template below with your actual request payload
//
// 3. Send the request and view the generated signature in the console
//    (The signature will be displayed in console)
// -----------------------------------------------------------
{
    "notifyType": "TXN",
    "transactionId": "1970492680761114624",
    "txnType": "REFUND",
    "merchantNo": "800391",
    "merchantTxnId": "1420171656261287936",
    "originMerchantTxnId": "1420171170523136000",
    "responseTime": "2025-09-23 22:20:37",
    "txnTime": "2025-09-23 22:20:35",
    "txnTimeZone": "+08:00",
    "orderAmount": "20.00",
    "orderCurrency": "PLN",
    "status": "S",
    "reason": "{\"respCode\":\"20000\",\"respMsg\":\"Success\"}",
    "sign": "8e698f6737c8987f17ded9b3294f80b86d0de1ee17d260df69d7b499d5a9edca",
    "paymentMethod": "BLIK",
    "channelRequestId": "8003911970493507072299009"
}'