Bypass Office 365 Sign In Page for external users

I am trying to find a solution to bypass Sign In page of Office 365 SharePoint with Custom sign in page and authenticate for external users.

I tried following code to perform authentication and it works but doesn't redirect to target SharePoint site. 

SharePointOnlineCredentials onlineCredentials = new SharePointOnlineCredentials(login, securePassword);

using (ClientContext clientContext = new ClientContext(targetSite))
{
Uri uri = new System.Uri(targetSite);
clientContext.Credentials = onlineCredentials;
Web web = clientContext.Web;
clientContext.Load(web,
webSite => webSite.Title);
clientContext.ExecuteQuery();
string authCookie = onlineCredentials.GetAuthenticationCookie(uri);
CookieContainer cookieContainer = new CookieContainer();
cookieContainer.SetCookies(uri, authCookie);
Response.Redirect(targetSite);
//Console.WriteLine(web.Title);
}

Any help or solution much appreciated.