SF_10.1, SF_10.2, SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/request-access-token-for-calling-web-services

StartupOwinMVCClient.cs

using Microsoft.Owin;
using Microsoft.Owin.Security.Cookies;
using Microsoft.Owin.Security.OpenIdConnect;
using Owin;
using System.Collections.Generic;
using System.IdentityModel.Tokens;
using System.Threading.Tasks;

[assembly: OwinStartup(typeof(MVCImplicitFlow.Startup))]
namespace MVCImplicitFlow
{
    public class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            JwtSecurityTokenHandler.InboundClaimTypeMap = new Dictionary<string, string>();

            app.UseCookieAuthentication(new CookieAuthenticationOptions
            {
                AuthenticationType = "Cookies"
            });

            app.UseOpenIdConnectAuthentication(new OpenIdConnectAuthenticationOptions
            {
                ClientId = "testApp",
                Authority = "http://yousitefinitsite/Sitefinity/Authenticate/OpenID",
                RedirectUri = "http://yoursite",
                ResponseType = "id_token token",
                Scope = "openid",

                UseTokenLifetime = false,
                SignInAsAuthenticationType = "Cookies",
                Notifications = new OpenIdConnectAuthenticationNotifications
                {
                    SecurityTokenValidated = n =>
                    {
                        var id = n.AuthenticationTicket.Identity;
                        id.AddClaim(new System.Security.Claims.Claim("access_token", n.ProtocolMessage.AccessToken));
                        n.AuthenticationTicket = new Microsoft.Owin.Security.AuthenticationTicket(id, n.AuthenticationTicket.Properties);
                        return Task.FromResult(0);
                    }
                }
            });
        }
    }
}