using System;

public class Program

{

    //https://docs.microsoft.com/en-us/azure/communication-services/tutorials/hmac-header-tutorial

    public static void Main()

    {

            string input = "{\"FormID\":\"4265104\",\"UniqueID\":\"857695328\",\"HandshakeKey\":\"IntegrationsSecret\",\"Short Answer\":\"Hi\",\"Dropdown List\":\"Building\"}";

            Console.WriteLine(ComputeSignature(ComputeContentHash(input)));

    }

    static string ComputeContentHash(string content)

    {

        using (var sha256 = System.Security.Cryptography.SHA256.Create())

        {

            byte[] hashedBytes = sha256.ComputeHash(System.Text.Encoding.UTF8.GetBytes(content));

            var toReturn = Convert.ToBase64String(hashedBytes);

            return System.Text.Encoding.UTF8.GetString(Convert.FromBase64String(toReturn));

        }

    }

​

    static string ComputeSignature(string stringToSign)

    {

        using (var hmacsha256 = new System.Security.Cryptography.HMACSHA256(Convert.FromBase64String("password")))

        {

            var bytes = System.Text.Encoding.ASCII.GetBytes(stringToSign);

            var hashedBytes = hmacsha256.ComputeHash(bytes);

            var toReturn = Convert.ToBase64String(hashedBytes);

            return System.Text.Encoding.UTF8.GetString(Convert.FromBase64String(toReturn));

        }

    }   

}