using System;

using System.Linq;

using System.Security.Cryptography;

using System.Text;

public class Program

{

    //https://www.freeformatter.com/hmac-generator.html#ad-output

    public static void Main()

    {

        string message = "{\"Topic\":\"FinancialTransactions\",\"ClientId\":\"4d172971-0ef1-46ac-93e2-cbeab649c103\",\"Division\":102,\"Action\":\"Update\",\"Key\":\"76368e4f-5495-4d9e-b1b4-079666bf3b59\",\"ExactOnlineEndpoint\":\"https://www.start.exactonline.nl/api/v1/102/financialtransaction/Transactions(guid'76368e4f-5495-4d9e-b1b4-079666bf3b59')\",\"EventCreatedOn\":\"2017-08-04T16:25:29.277\"}";

        string secret = "TuJBuL8GrCXs";

        Console.WriteLine(GetHMAC256HexString(message, secret));

    }

    public static string GetHMAC256HexString(string message, string secretKey)

        {

            ASCIIEncoding encoding = new ASCIIEncoding();

            byte[] keyByte = encoding.GetBytes(secretKey);

​

            byte[] messageBytes = encoding.GetBytes(message);

​

            HMACSHA256 hmac = new HMACSHA256(keyByte);

​

            byte[] hashMessage = hmac.ComputeHash(messageBytes);

​

            string hexString = string.Concat(hashMessage.Select(x => x.ToString("X2")));

​

            return hexString;

        }

}