using System;

using System.Net.Http;

using System.Threading.Tasks;

​

public class Program

{

    public static async Task Main()

    {

        var httpClient = new HttpClient();

        var funkyUserAgent = "Dalvik/2.1.0 (Linux; U; Android 5.1.1; YAL-AL10 Build/LMY48Z) [something]";

        var httpRequestMessage = new HttpRequestMessage(HttpMethod.Get, "https://postman-echo.com/headers");

        httpRequestMessage.Headers.TryAddWithoutValidation("User-Agent", funkyUserAgent);

        var result = await httpClient.SendAsync(httpRequestMessage);

        var content = await result.Content.ReadAsStringAsync();

        Console.WriteLine(content);

    }

}