using System;

using System.Text;

using System.Security.Cryptography;

public class Program

{

    public static void Main()

    {

        var secretKey = "F4147E77941DB593C991F9A3C7C34";

        var signatureString = "{\"id\":51404608,\"order\":{\"id\":11518977,\"delivery_address\":{\"id\":2599937,\"firstname\":\"Gijs\",\"lastname\":\"Ros\",\"street\":\"De Acacia\",\"house_number\":\"10\",\"zipcode\":\"6941DC\",\"city\":\"DIDAM\",\"country\":\"NL\"},\"email\":\"gijs.ros@marketingrocks.nl\",\"external_id\":\"235\",\"order_date\":\"2020-06-12T16:59:45+02:00\",\"order_number\":\"235\",\"order_lines\":[{\"id\":3815592,\"article\":{\"id\":826270,\"code\":\"STIM 91471\"},\"quantity\":1}],\"partner\":{\"code\":\"STIM\",\"name\":\"Stimulan B.V.\"},\"state\":15},\"state\":15,\"shipment_lines\":[{\"id\":3778486,\"quantity\":1,\"order_line\":{\"id\":3815592,\"article\":{\"id\":826270,\"code\":\"STIM 91471\"},\"quantity\":1}}]}";

        var enc = Encoding.ASCII;

        HMACSHA1 hmac = new HMACSHA1(enc.GetBytes(secretKey));

        hmac.Initialize();

​

        byte[] buffer = enc.GetBytes(signatureString);

        var calculatedSignature =  BitConverter.ToString(hmac.ComputeHash(buffer)).Replace("-", "").ToLower();

        Console.WriteLine(calculatedSignature);

    }

}