using System;

using Microsoft.AspNetCore.Http;

public class Program

{

    public static void Main()

    {

        var email = "jbatte@gmail.com";

        var builder = new UriBuilder("https://www.google.com");

        builder.Query = QueryString.Create("$filter", $"mail eq '{email}'").Value;

        Console.WriteLine(builder.Uri);

    }

}

​