Azure AD authentication in ASP.NET web forms using OWIN

Try
          System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls11 Or SecurityProtocolType.Tls12
          Dim redirectUri As String = ConfigurationManager.AppSettings("ida:RedirectUri")
          Dim authProperties As New AuthenticationProperties
          authProperties.RedirectUri = redirectUri
          HttpContext.Current.GetOwinContext().Authentication.Challenge(authProperties, OpenIdConnectAuthenticationDefaults.AuthenticationType)
      Catch ex As Exception
          LblErrorMsg.Text = ex.Message
          PnlError.Visible = True
          Exit Sub
      End Try

Imports Microsoft.Owin.Security.OpenIdConnect
Imports Microsoft.Owin.Security.Notifications
Imports System.Threading.Tasks
Imports System
Imports Microsoft.Owin.Host.SystemWeb

<Assembly: OwinStartup("MyStartupClass", GetType(Startup))>
Public Class Startup
    Private clientId As String = System.Configuration.ConfigurationManager.AppSettings("ida:ClientId")
    Private redirectUri As String = System.Configuration.ConfigurationManager.AppSettings("ida:RedirectUri")
    Shared tenant As String = System.Configuration.ConfigurationManager.AppSettings("ida:Tenant")
    Private authority As String = String.Format(System.Globalization.CultureInfo.InvariantCulture, System.Configuration.ConfigurationManager.AppSettings("ida:Authority"), tenant)

    Public Sub Configuration(ByVal app As IAppBuilder)
        app.SetDefaultSignInAsAuthenticationType(CookieAuthenticationDefaults.AuthenticationType)
        app.UseCookieAuthentication(New CookieAuthenticationOptions With {
            .CookieManager = New SystemWebCookieManager()
        })
        app.UseOpenIdConnectAuthentication(New OpenIdConnectAuthenticationOptions With {
            .ClientId = clientId,
            .Authority = authority,
            .RedirectUri = redirectUri,
            .PostLogoutRedirectUri = redirectUri,
            .Scope = OpenIdConnectScope.OpenIdProfile,
            .ResponseType = OpenIdConnectResponseType.IdToken,
            .TokenValidationParameters = New TokenValidationParameters() With {
                .ValidateIssuer = False
            },
            .Notifications = New OpenIdConnectAuthenticationNotifications With {
                .AuthenticationFailed = AddressOf OnAuthenticationFailed
            }
        })
    End Sub

    Private Function OnAuthenticationFailed(ByVal context As AuthenticationFailedNotification(Of OpenIdConnectMessage, OpenIdConnectAuthenticationOptions)) As Task
        context.HandleResponse()
        context.Response.Redirect("/?errormessage=" & context.Exception.Message)
        Return Task.FromResult(0)
    End Function
End Class

Dim name As String = ""

       If HttpContext.Current.GetOwinContext().Authentication.User.Identity.IsAuthenticated = True Then
           name = HttpContext.Current.GetOwinContext().Authentication.User.Identity.Name
       End If