public IActionResult OnPost(string provider, string returnUrl = null)
{
// Request a redirect to the external login provider.
var redirectUrl = Url.Page("./ExternalLogin", pageHandler: "Callback", values: new { returnUrl });
var properties = _externalLoginService.CreateAuthenticationProperties(provider, redirectUrl);
return(new ChallengeResult(provider, new AuthenticationProperties(properties)));
}