using System;

using System.Net;

using System.Security;

public class Program

{

    public static void Main()

    {

        Uri value = new Uri("https://ai.mmmlabs.com/whatever?foo=bar&boo=baz");

        var built = BuildURLWithData(value, "yo this is some great data");

        Console.WriteLine(built);

    }

    private static Uri BuildURLWithData(Uri uri, string data)

        {

            if(!uri.IsAbsoluteUri)

            {

                throw new ArgumentException("URL must be absolute.");

            }

​

            if(uri.Scheme != "https")

            {

                throw new ArgumentException("URL must be https.");

            }

​

            byte[] dataAsUtf8 = System.Text.Encoding.UTF8.GetBytes(data);

            string dataQueryString = $"data={WebUtility.UrlEncode(Convert.ToBase64String(dataAsUtf8))}";

            UriBuilder builder = new UriBuilder(uri);

            if(builder.Query.Length > 1)

                // Note: In .NET Core and .NET 5+, you can simplify by removing

                // the call to Substring(), which removes the leading "?" character.

            {

                builder.Query = $"{builder.Query.Substring(1)}&{dataQueryString}";

            }

            else

            {

                builder.Query = dataQueryString;

            }

​

            Uri finalUri = builder.Uri;

            return finalUri;

        }

}