oAuth2 web request works in browser but not in app - azure

Question

I have the following code sample with which I'm trying to authenticate an Azure active directory user within a Xamarin forms app
The URL (I've removed the actual client ID) works fine in a browser but fails when trying to send the http request
The error message says 'the response type must include client_id'
string URL = "https://login.microsoftonline.com/common/oauth2/v2.0/authorize?"
+ "client_id=xxxx-xxxxx-xxxxx-xxxxx-xxx"
+ "&response_type=code"
+ "&redirect_uri=https://login.microsoftonline.com/common/oauth2/nativeclient"
+ "&response_mode=query"
+ "&scope=openid%20offline_access%20https%3A%2F%2Fgraph.microsoft.com%2Fmail.read"
+ "&state=12345";
var webRequest = System.Net.WebRequest.Create(URL) as HttpWebRequest;
System.Console.WriteLine(URL);
if (webRequest != null)
{
webRequest.Method = "POST";
webRequest.ServicePoint.Expect100Continue = false;
webRequest.Timeout = 20000;
webRequest.ContentType = "text/html";
//POST the data.
using (requestWriter = new StreamWriter(webRequest.GetRequestStream()))
{
requestWriter.Write(postData);
}
}
HttpWebResponse resp = (HttpWebResponse)webRequest.GetResponse();
Stream resStream = resp.GetResponseStream();
StreamReader reader = new StreamReader(resStream);
ret = reader.ReadToEnd();

Answer 1

You put parameters in the URL, so you need to use GET method, instead of POST (like your browser does when you paste the URL in its address bar).
So, replace:
webRequest.Method = "POST";
by:
webRequest.Method = "GET";
and remove:
//POST the data.
using (requestWriter = new StreamWriter(webRequest.GetRequestStream()))
{
requestWriter.Write(postData);
}