protected void btn_Click(object sender, EventArgs e)
{
//We need to get user's selected provider enumerator.
PROVIDER_TYPE selectedProvider = (PROVIDER_TYPE)Enum.Parse(typeof(PROVIDER_TYPE), ((Button)sender).Text.ToUpper());
//Initialize User
SocialAuthUser objUser = new SocialAuthUser(selectedProvider);
//Call Login
objUser.Login(returnUrl: "ManualLogin.aspx");
//Login method also accepts a parameter for URL to which user should be redirected after login. If not specified,
//automatically defaultUrl as set in Web.Config will be picked for redirection.
}