using System;

using System.Text;

using System.Linq;

using System.Collections.Generic;

using System.Net.Http;

using System.Threading.Tasks;

public class Program

{

    public static async Task Main()

    {

        var formData = new KeyValuePair<string, string>[] { 

            new KeyValuePair<string, string>("@@FirstName@@firs", "a"),

            new KeyValuePair<string, string>("@@LastName@@last", "b1"),

            new KeyValuePair<string, string>("@@Address1@@addre", "C"),

        };

        //Gag Baghdasaryan's suggested solution

        var requestData = new FormUrlEncodedContent(formData);

        var contentAsString = await requestData.ReadAsStringAsync();

        contentAsString.Dump();

        //Peter Csala's solution

        static string Encode(string data) => Uri.EscapeDataString(data).Replace("%20", "+");

        var formDataString = formData.Aggregate(new StringBuilder(), 

                                                (sb,kv) => sb.Append($"&{Encode(kv.Key)}={Encode(kv.Value)}"), 

                                                (sb) => sb.Remove(0,1).ToString());

        formDataString.Dump();

    }

}