Try this (this uses a native .net library) - I'm using .net core 3.1 but I believe it should work in other versions. You need Json.Net to parse the answer. This uses oAuth 2.0 as 1.0 has been deprecated.
dynamic results;
string url = @"https://api.dialogflow.com/v1/intents/<intentId>";
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
request.Headers.Add(HttpRequestHeader.Authorization, "Bearer <your dialogflow developer key>");
try
{
WebResponse response = request.GetResponse();
using (Stream responseStream = response.GetResponseStream())
{
StreamReader reader = new StreamReader(responseStream, System.Text.Encoding.UTF8);
var restResponse = reader.ReadToEnd();
results = JsonConvert.DeserializeObject<dynamic>(restResponse);
}
catch (WebException ex)
{
WebResponse errorResponse = ex.Response;
using (Stream responseStream = errorResponse.GetResponseStream())
{
StreamReader reader = new StreamReader(responseStream, System.Text.Encoding.GetEncoding("utf-8"));
String errorText = reader.ReadToEnd();
}
throw;
}