Webhook validation URL not formed correctly

I am trying to setup a webhook for an Adobe IO event. It is failing on the validation step. For the asyncronous validation (https://www.adobe.io/apis/experienceplatform/events/docs.html#!adobedocs/adobeio-events/master/intro/webhooks_intro.md#asynchronous-validation), I am getting a URL that doesn't appear to be formed correctly. There is an example below where there are XXXXs in the url. Should the XXXXs be in the URL? The example in the documentation does not include them.

{"validationUrl":"https://csm.adobe.io/csm/registrations/validate?id=<hiding id>&challenge=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXX6782"}