using System;

using System.Collections.Generic;

using System.Linq;

using Microsoft.AspNetCore.WebUtilities;

using Microsoft.AspNetCore.Http.Extensions;

public class Program

{

    public static void Main()

    {

        var id = Guid.NewGuid();

        var urlWithExistingQuery = "https://trustpay.sk/blabla?param=xyz";

        var dirtyUrlWithExistingQuery = QueryHelpers.AddQueryString(urlWithExistingQuery, "instanceId", id.ToString());

        System.Console.WriteLine(dirtyUrlWithExistingQuery);

    }

}