Azure Authentication using OAuth with token in ASP.NET WebForms NOT MVC

I want to implement Azure authentication for my WebForms using OAuth (It is working for me). After authentication I required token for validate in server side and it will validate in every client code. But I am not getting Token after authentication

I have created a startup class and configure it using owin and it authenticating correctly but not able to get the token after authentication.

Startup class

using Microsoft.Owin;
using Owin;
using Microsoft.IdentityModel.Protocols.OpenIdConnect;
using Microsoft.IdentityModel.Tokens;
using Microsoft.Owin.Security;
using Microsoft.Owin.Security.Cookies;
using Microsoft.Owin.Security.OpenIdConnect;
using Microsoft.Owin.Security.Notifications;
using System;
using System.Threading.Tasks;

[assembly: OwinStartup(typeof(Manager.Startup))]

namespace Manager
{
    public class Startup
    {
        // The Client ID is used by the application to uniquely identify itself to Azure AD.
        string clientId = System.Configuration.ConfigurationManager.AppSettings["ClientId"];

        // RedirectUri is the URL where the user will be redirected to after they sign in.
        string redirectUri = System.Configuration.ConfigurationManager.AppSettings["RedirectUri"];

        // Tenant is the tenant ID (e.g. contoso.onmicrosoft.com, or 'common' for multi-tenant)
        static string tenant = System.Configuration.ConfigurationManager.AppSettings["Tenant"];

        // Authority is the URL for authority, composed by Azure Active Directory v2 endpoint and the tenant name (e.g. https://login.microsoftonline.com/contoso.onmicrosoft.com/v2.0)
        string authority = String.Format(System.Globalization.CultureInfo.InvariantCulture, System.Configuration.ConfigurationManager.AppSettings["Authority"], tenant);

        /// <summary>
        /// Configure OWIN to use OpenIdConnect 
        /// </summary>
        /// <param name="app"></param>
        public void Configuration(IAppBuilder app)
        {
            app.SetDefaultSignInAsAuthenticationType(CookieAuthenticationDefaults.AuthenticationType);

            app.UseCookieAuthentication(new CookieAuthenticationOptions());
            app.UseOpenIdConnectAuthentication(
                new OpenIdConnectAuthenticationOptions
                {
                // Sets the ClientId, authority, RedirectUri as obtained from web.config
                ClientId = clientId,
                    Authority = authority,
                    RedirectUri = redirectUri,
                // PostLogoutRedirectUri is the page that users will be redirected to after sign-out. In this case, it is using the home page
                PostLogoutRedirectUri = redirectUri,
                    Scope = OpenIdConnectScope.OpenIdProfile,
                // ResponseType is set to request the id_token - which contains basic information about the signed-in user
                ResponseType = OpenIdConnectResponseType.IdToken,
                // ValidateIssuer set to false to allow personal and work accounts from any organization to sign in to your application
                // To only allow users from a single organizations, set ValidateIssuer to true and 'tenant' setting in web.config to the tenant name
                // To allow users from only a list of specific organizations, set ValidateIssuer to true and use ValidIssuers parameter 
                TokenValidationParameters = new TokenValidationParameters()
                    {
                        ValidateIssuer = false
                    },
                // OpenIdConnectAuthenticationNotifications configures OWIN to send notification of failed authentications to OnAuthenticationFailed method
                Notifications = new OpenIdConnectAuthenticationNotifications
                    {
                        AuthenticationFailed = OnAuthenticationFailed
                    }
                }
            );
        }

        /// <summary>
        /// Handle failed authentication requests by redirecting the user to the home page with an error in the query string
        /// </summary>
        /// <param name="context"></param>
        /// <returns></returns>
        private Task OnAuthenticationFailed(AuthenticationFailedNotification<OpenIdConnectMessage, OpenIdConnectAuthenticationOptions> context)
        {
            context.HandleResponse();
            context.Response.Redirect("/?errormessage=" + context.Exception.Message);
            return Task.FromResult(0);
        }
    }
}

on page load

 protected void Page_Load(object sender, EventArgs e)
        {
            if (!Request.IsAuthenticated)
            {
                Context.GetOwinContext().Authentication.Challenge(new AuthenticationProperties { RedirectUri = "/" },
        OpenIdConnectAuthenticationDefaults.AuthenticationType);

// below codes are tested but all are giving null or empty string 

                var claimsIdentity = User.Identity as ClaimsIdentity;
                string str = claimsIdentity?.Claims.FirstOrDefault(c => c.Type == ClaimTypes.Name)?.Value;
                string accessToken = claimsIdentity?.Claims.FirstOrDefault(c => c.Type == "access_token")?.Value;
                string idToken = claimsIdentity?.Claims.FirstOrDefault(c => c.Type == "id_token")?.Value;
                string refreshToken = claimsIdentity?.Claims.FirstOrDefault(c => c.Type == "refresh_token")?.Value;
                str = User.Identity.Name;
                GetTokenForApplication().Wait(); ;
            }

        }

Expected result is Token after authentication Actual result is not getting token after authentication