I am trying to register an Autodesk APS (Forge) webhook for the Model Derivative extraction.finished event from my .NET MVC 4 application.

I register the webhook using:

POST https://developer.api.autodesk.com/webhooks/v1/systems/derivative/events/extraction.finished/hooks

Body:

{
  "callbackUrl": "<>",
  "scope": {
    "workflow": "modelderivative"
  }
}

The API returns 201 Created, and I can see the webhook listed in the Hooks GET API.
However, my callback endpoint never receives any webhook calls.

The callback URL works in a browser

When I visit:

<>?challenge=123

my MVC controller receives the request and correctly returns 123.

But Autodesk never reaches my server

I added a debugger and logs to the controller:

public ActionResult Callback()
{
    if (Request.HttpMethod == "GET")
    {
        var challenge = Request.QueryString["challenge"];
        return Content(challenge);
    }

    Request.InputStream.Position = 0;
    using (var reader = new StreamReader(Request.InputStream))
    {
        var body = reader.ReadToEnd();
        // log body
    }

    return new HttpStatusCodeResult(200);
}

The debugger does hit when I call the URL from a browser.
But Autodesk never triggers the GET challenge or POST event.

My Model Derivative translation job

I start the translation using:

POST https://developer.api.autodesk.com/modelderivative/v2/designdata/job

Body:

{
  "input": {
    "urn": "<>"
  },
  "output": {
    "formats": [
      {
        "type": "svf2",
        "views": [ "2d", "3d" ]
      }
    ],
    "misc": {
      "workflow": "modelderivative"
    }
  }
}

The translation succeeds (verified via the manifest API),
but no webhook is ever delivered to my callback URL.