Programmatically downloading file

Using C# 4.5,I'm trying to programmatically log-on to a third-party website and then download a csv file from another page on that site. I tried using following approach.I dont get any errors but it seems to download the contents of their logon page and save it as a csv file instead of the actual csv file having data.

Also,if I read the response stream after logging in, I see that I'm getting the log-on webpage back.

What am I missing here please? Plus, I need to post the name of the form as well.How do I achieve that?Please advise.

Update:I tried using CookieContainer in request object, but still no luck..I get the same result.

Thanks.

//Log-on to website and post user and password

var request = new HttpRequestMessage(HttpMethod.Post, myLogonURL);

request.Content = new StringContent("{\"user\":\"abcd\",\"password\":test}", Encoding.UTF8, "application/x-www-form-urlencoded");

var sendTask = client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);

var response = sendTask.Result.EnsureSuccessStatusCode();

//Download the file

request = new HttpRequestMessage(HttpMethod.Get, fileDownloadURL);

sendTask = client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);

response = sendTask.Result.EnsureSuccessStatusCode();

var httpStream = await response.Content.ReadAsStreamAsync();

//Save the downloaded file

using (var fileStream = File.Create(downloadedFilePath))

using (var reader = new StreamReader(httpStream)) {

httpStream.CopyTo(fileStream);

fileStream.Flush();

}