using (var httpClient = new HttpClient())
{
httpClient.BaseAddress = new Uri("http://somesite.com");
var content = new FormUrlEncodedContent(new[]
{
new KeyValuePair<string, string>("accountidentifier", accountID),
new KeyValuePair<string, string>("type", "add"),
new KeyValuePair<string, string>("seriesid", seriesId),
});
httpClient.PostAsync("/api/User_Favorites.php", content);
}