public async Task <string> GetToken(string code, string redirectUrl)
        {
            var appId     = configuration["Authentication:Facebook:AppId"];
            var appSecret = configuration["Authentication:Facebook:AppSecret"];

            var result = await facebookClient.GetPublicAsync <dynamic>(
                "oauth/access_token",
                $"client_id={appId}&redirect_uri={redirectUrl}&client_secret={appSecret}&code={code}");

            if (result == null)
            {
                return(null);
            }

            return(result.access_token);
        }