From a class, how do you modify a string from the GUI?

using BasicProject.Model;
using Newtonsoft.Json;
using System.Collections.Generic;
using System.Net.Http;
using System.Threading.Tasks;

public class BinanceApi
{
    public static async Task<List<UserRank>> 
       GetLeaderboardRank(string tradeType)
    {
        try
        {
            using (HttpClient client = new HttpClient
                  ((HttpMessageHandler)new HttpClientHandler()
            {
                UseCookies = false
            }))
            {
                string payload = JsonConvert.SerializeObject((object)new
                {
                    tradeType = tradeType,
                    isShared = true,
                    isTrader = false,
                    periodType = "MONTHLY", 
                    statisticsType = "ROI"
                });
            }
            return userRanks;
         }
    }
    catch (Exception ex)
    {
        BinanceApi.Log.Error((object) ex);
        return (List<UserRank>)null;
    }
}