Trying to add a webhook returns Bad Request

We're trying to integrate with the Reach 360 api.  I've tried following their documentation on creating a webhook to be notified when a learner completes a course, but when I try to post the following json to the create endpoint I get a bad request response.  Support said they can't troubleshoot it and to ask here, so hopefully someone might have an idea.

{
    "id": "",
    "url": "",
    "targetUrl": "https://ourwebapp.com/callbackreach360",
    "events": [
        "course.completed"
    ]
}

And this is the documentation I'm following - https://access.articulate.com/support/article/Reach-360-Webhooks-API#create

Thanks.