How to use HttpClient to Post with Authentication

First you have to set the Authorization-Header with your <clientid> and <clientsecret>.

Instead of using StringContent you should use FormUrlEncodedContent as shown below:

var client = new HttpClient();
client.BaseAddress = new Uri("http://myserver");
var request = new HttpRequestMessage(HttpMethod.Post, "/path");

var byteArray = new UTF8Encoding().GetBytes("<clientid>:<clientsecret>");
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", Convert.ToBase64String(byteArray));

var formData = new List<KeyValuePair<string, string>>();
formData.Add(new KeyValuePair<string, string>("grant_type", "password"));
formData.Add(new KeyValuePair<string, string>("username", "<email>"));
formData.Add(new KeyValuePair<string, string>("password", "<password>"));
formData.Add(new KeyValuePair<string, string>("scope", "all"));

request.Content = new FormUrlEncodedContent(formData);
var response = await client.SendAsync(request);