title: How to pass parameter to fb auth callback in ASP.NE Core 2.1, without identity?
body: <p>How I can to pass parameter to Challenge method, and read it after facebook callback? I want to link social accounts, and id can be user id for example.</p>
  
  <p>Account controller example:</p>
  
  <pre><code>    [Route("accounts/facebook/{id}")]
      public IActionResult Facebook(string id)
      {
          var auth = new AuthenticationProperties { RedirectUri = "/accounts/facebook/callback" };
  
          return Challenge(auth, "Facebook");
      }
  
      [Route("accounts/facebook/callback")]
      public void Callback() 
      {
          //need to read custumId
      }
  </code></pre>
  
  <p>Startup.cs example:</p>
  
  <pre><code> public void ConfigureServices(IServiceCollection services)
      {
          services
              .AddAuthentication()
              .AddFacebook(x =&gt;
              {
                  x.AppId = "*";
                  x.AppSecret = "*";
                  x.CallbackPath = "/accounts/facebook/callback";
              });
  
          services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
      }
  
      public void Configure(IApplicationBuilder app, IHostingEnvironment env)
      {
          app.UseDeveloperExceptionPage();
  
          app.UseMvc(routes =&gt;
          {
              routes.MapRoute(
                  name: "default",
                  template: "{controller=Home}/{action=Index}/{id?}");
          });
      }
  </code></pre>
  
  <p>Summary, I need to link social accounts to exiting profile in my system.</p>