PostBuffer.cs

var baseAddress = new Uri(API_URL_BASE);
var cookieContainer = new CookieContainer();
using (var handler = new HttpClientHandler() { CookieContainer = cookieContainer })
using (var client = new HttpClient(handler) { BaseAddress = baseAddress })
{
    // some defaults, add a header and cookie for authenticating with the API
    client.DefaultRequestHeaders.Add("User-Agent", USER_AGENT);
    client.DefaultRequestHeaders.Add("X-AvoSig", AvoSignature);
    cookieContainer.Add(baseAddress, new Cookie(COOKIE_NAME, CookieValue));
    // Post the content and return the result
    var response = client.PostAsync(uri, content);
    return response.Result;
}