Adding Webhook to SharePoint List

Question

I'm using Postman to add a Webhook to a SharePoint List and am having an permission issue:

POST https://[Company].sharepoint.com/sites/Test/_api/web/lists('80105E8F-2CEA-4D49-8238-2A1829B46494')/subscriptions

Body:
{
  "resource": "https://[Company].sharepoint.com/sites/Test/_api/web/lists('80105E8F-2CEA-4D49-8238-2A1829B46494')",
  "notificationUrl": "https://MyFunctions.azurewebsites.net/api/HttpTriggerCSharp1?code=[ClientCode]",
  "expirationDateTime": "2017-09-27T16:17:57+00:00"
}

Response:
{
    "error": {
        "code": "-2147024891, System.UnauthorizedAccessException",
        "message": {
            "lang": "en-US",
            "value": "Access denied. You do not have permission to perform this action or access this resource."
        }
    }
}

I'm the SharePoint Site Collection Administrator and I can successfully list the current webhooks (there are none). I'm listing the current webhooks with the following command:

GET https://[Company].sharepoint.com/sites/Test/_api/web/lists('80105E8F-2CEA-4D49-8238-2A1829B46494')/subscriptions

Response:
{
    "d": {
        "results": []
    }
}