public void Main()
{
HttpWebRequest endpointRequest =
(HttpWebRequest)HttpWebRequest.Create("https://domainname.sharepoint.com/_api/web/currentuser/?$expand=groups");
endpointRequest.Method = "GET";
endpointRequest.Accept = "application/json;odata=verbose";
NetworkCredential cred = new System.Net.NetworkCredential("userName", "Password", "domain name");
endpointRequest.Credentials = cred;


        try
        {
            HttpWebResponse endpointResponse = (HttpWebResponse)endpointRequest.GetResponse();
            WebResponse webResponse = endpointRequest.GetResponse();
            Stream webStream = webResponse.GetResponseStream();
            StreamReader responseReader = new StreamReader(webStream);
            string response = responseReader.ReadToEnd();
            MessageBox.Show(response);
        }
        catch (Exception e)
        {
            MessageBox.Show(e.Message);
            //Console.Out.WriteLine(e.Message); Console.ReadLine();
        }

    }

string siteUrl = "https://tenant.sharepoint.com/sites/developer/";
            string userName = "lee@tenant.onmicrosoft.com";
            string password = "password";

            using (var ctx = new ClientContext(siteUrl))
            {
                SecureString securePassword = new SecureString();
                foreach (char c in password.ToCharArray()) securePassword.AppendChar(c);
                ctx.Credentials = new SharePointOnlineCredentials(userName, securePassword);

                string sHtml = "";
                HttpWebRequest getRequest;
                HttpWebResponse response = null;
                Stream stream = null;

                var uri = new Uri("https://tenant.sharepoint.com/sites/Developer/_api/web/currentuser/?$expand=groups");
                var credentials = new SharePointOnlineCredentials(userName, securePassword);
                var authCookie = credentials.GetAuthenticationCookie(uri);
                var cookieContainer = new CookieContainer();
                cookieContainer.SetCookies(uri, authCookie);

                getRequest = (HttpWebRequest)WebRequest.Create(uri);
                getRequest.CookieContainer = cookieContainer;
                getRequest.Credentials = credentials;
                getRequest.Proxy.Credentials = System.Net.CredentialCache.DefaultCredentials;
                getRequest.Method = "GET";
                getRequest.Accept = "application/json;odata=verbose";

                response = (HttpWebResponse)getRequest.GetResponse();
                stream = response.GetResponseStream();
                StreamReader responseReader = new StreamReader(stream);
                string _Response = responseReader.ReadToEnd();
                Console.ReadKey();
            }