The demo code uses the Async features built into .NET 4.5.

Connecting Via Query String Authentication

void TestConnectingViaQueryStringAuthentication()
{
    var client = new HttpClient() { BaseAddress = new Uri("https://api.oneguard.net/") };
    var userName = "username";
    var password = "password";
 
    var response = client.GetAsync(string.Format("api/Test/ShouldJustWork?username={0}&password={1}", userName, password)).Result;
    response.EnsureSuccessStatusCode();
 
    var contentResponse = response.Content.ReadAsStringAsync();
    Console.WriteLine(contentResponse);
}