var url = new Uri("http://localhost:8090/rest/auth/latest/session?os_username=tempusername&os_password=temppwd");
var request = WebRequest.Create(url) as HttpWebRequest;
if (null == request)
{
 return "";
}
request.Method = "POST";
request.ContentType = "application/json";
request.ContentLength = 200;
request.KeepAlive = false;
using (var response = request.GetResponse() as HttpWebResponse)
{
}

public enum JiraResource
{
    project
}

protected string RunQuery(
    JiraResource resource,
    string argument = null,
    string data = null,
    string method = "GET")
{
    string url = string.Format("{0}{1}/", m_BaseUrl, resource.ToString());

    if (argument != null)
    {
        url = string.Format("{0}{1}/", url, argument);
    }

    HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
    request.ContentType = "application/json";
    request.Method = method;

    if (data != null)
    {
        using (StreamWriter writer = new StreamWriter(request.GetRequestStream()))
        {
            writer.Write(data);
        }
    }

    string base64Credentials = GetEncodedCredentials();
    request.Headers.Add("Authorization", "Basic " + base64Credentials);

    HttpWebResponse response = request.GetResponse() as HttpWebResponse;

    string result = string.Empty;
    using (StreamReader reader = new StreamReader(response.GetResponseStream()))
    {
        result = reader.ReadToEnd();
    }

    return result;
}

private string GetEncodedCredentials()
{
    string mergedCredentials = string.Format("{0}:{1}", m_Username, m_Password);
    byte[] byteCredentials = UTF8Encoding.UTF8.GetBytes(mergedCredentials);
    return Convert.ToBase64String(byteCredentials);
}

var mergedCredentials = string.Format("{0}:{1}", username, password);
var byteCredentials = Encoding.UTF8.GetBytes(mergedCredentials);
var encodedCredentials = Convert.ToBase64String(byteCredentials);

using (WebClient webClient = new WebClient())
{
    webClient.Headers.Set("Authorization", "Basic " + encodedCredentials);

    return webClient.DownloadString(url);
}

public async Task<JiraCookie> GetCookieAsync(string myJsonPass, string JiraCookieEndpointUrl)
{
    using (var client = new HttpClient())
        {
            var response = await client.PostAsync(
                JiraCookieEndpointUrl,
                new StringContent(myJsonPass, Encoding.UTF8, "application/json"));
            var json = response.Content.ReadAsStringAsync().Result;
                var jiraCookie= JsonConvert.DeserializeObject<JiraCookie>(json);
                return jArr;
         }
}

public class JiraCookie
{
    public Session session { get; set; }
}

public class Session
{
    public string name { get; set; }
    public string value { get; set; }
}

{
"session" : -{
"name" : JSESSIONID,
"value" : cookieValue
}