Output.txt

foo=bar%26foo&lorem=ipsum&bar=bar
Press any key to continue . . .

QueryString.cs

var content = new FormUrlEncodedContent(
    new Dictionary<string, string>
    {
        ["foo"] = "bar&foo",
        ["lorem"] = "ipsum",
        ["bar"] = "bar"
    });

Console.WriteLine(content.ReadAsStringAsync().Result);