using System;

using System.Text;

using System.Security.Cryptography;

public class Program

{

    public static void Main()

    {

        var secret = "testv4$1234";

        var payload = "{\"monitorId\":\"70817243-51d4-4bc0-bb12-393cb8bc05ce\",\"billingCodeId\":\"a5e748a6-a49f-4fbd-85ab-5bd912454faf\"}";

        ComputeHashTU(secret, payload);

    }

    private static void ComputeHashTU(string secret, string payload)

    {

        using (var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(secret)))

        { 

            byte[] computedHash = hmac.ComputeHash(Encoding.UTF8.GetBytes(payload));

            string hexString = BitConverter.ToString(computedHash).Replace("-", "").ToLowerInvariant();

            Console.WriteLine(hexString);

        }

     }

}