using System;

using System.Text;

using System.Security.Cryptography;

public class Program

{

    private const string key = "8b4bd06b1f4ca023f72f3aba95300cd2";

    private const string message = "id_user=910&time=1585914710";

    private static readonly Encoding encoding = Encoding.UTF8; 

    public void Main(string[] args)

    {

        Console.WriteLine("Result: {0}", Base64Encode(BitConverter.ToString(hmacSHA256(message, key)).Replace("-", "").ToLower()));

​

    }

    static byte[] hmacSHA256(String data, String key)

    {

        using (HMACSHA256 hmac = new HMACSHA256(Encoding.ASCII.GetBytes(key)))

        {

            return hmac.ComputeHash(Encoding.ASCII.GetBytes(data));

        }

    }

    static string ByteToString(byte[] buff)

    {

        string sbinary = "";

        for (int i = 0; i < buff.Length; i++)

            sbinary += buff[i].ToString("X2"); /* hex format */

        return sbinary;

    }

    public static string Base64Encode(string plainText) {

      var plainTextBytes = System.Text.Encoding.UTF8.GetBytes(plainText);

      return System.Convert.ToBase64String(plainTextBytes);

    }

}