void Main()
{
func().Wait();
}

async Task func()
{
var Content = new FormUrlEncodedContent(new Dictionary<string, string>
{ { "form_sent", "1" },
{ "redirect_url", "" },
{ "req_username", "user" },
{ "req_password", "user" },
{ "login", "%C2%EE%E9%F2%E8" } });

var client = new HttpClient();

var response = await client.PostAsync("http://landwatersun.ru/login.php?action=in", Content);
string responseString = await response.Content.ReadAsStringAsync();
Console.WriteLine(response.StatusCode);

using (StreamWriter sw = new StreamWriter(@"c:\Users\landw\Downloads\code.html", false, System.Text.Encoding.Default))
{
    sw.Write(responseString);
}
}