Sitecore Azure AD B2C User.Identity.IsAuthenticated is false Set AuthenticationManager

externallogincallback.cs

 /*
  Reference
   https://blog.baslijten.com/federated-authentication-in-sitecore-error-unsuccessful-login-with-external-provider/
 */
 public void SignIn(string returnUrl)
 {
     // The param returnUrl is configured in Azure B2C Identity Provider.
     // The redirect URL is to redirect to externallogincallback method of Sitecore Owin and then load returnUrl Sign-in
     var properties = new AuthenticationProperties() { RedirectUri = "https://abc/identity/externallogincallback?ReturnUrl=/sign-in" };
     
     //retain returnUrl to later redirect user to the page they first started            
     if(!string.IsNullOrWhiteSpace(returnUrl))
         properties.Dictionary["returnUrl"] = returnUrl;

     System.Web.HttpContext.Current.GetOwinContext().Authentication.Challenge(properties,
         new string[] { "AzureB2C", "SignUpSignInPolicyKey" });
 }