I have a legacy asp.net web forms application built in .net framework 4.8 Recently it is being upgraded to use Azure AD Open Id authentication. I used Microsoft Owin Project Git project as reference to configure the authentication. Authentication is working fine. I am able to retrieve user claims as well. However, in my project there is no login page. If user tries to access any page & he is not authenticated then it should direct to the IdP authority url for login. In a sample project below code is used to redirect user to IdP login page.

if (!Request.IsAuthenticated)
{
    HttpContext.Current.GetOwinContext().Authentication.Challenge(
        new AuthenticationProperties { RedirectUri = "/" },
        OpenIdConnectAuthenticationDefaults.AuthenticationType);
}

This is what I tried:

<authentication mode="None"></authentication>
<authorization>
  <deny users="?" />
</authorization>

In global.asax

protected void Application_AuthenticateRequest(Object sender, EventArgs e)
     {
         if (!Request.IsAuthenticated)
         {
             HttpContext.Current.GetOwinContext().Authentication.Challenge(
                 new AuthenticationProperties { RedirectUri = "/" },
                 OpenIdConnectAuthenticationDefaults.AuthenticationType);
         }
     }

But user is not redirected to IdP login. What should I change here to implement this?