Getting User Custom Fields to Form Fields

I am trying to copy custom user fields into a custom ticket fields via a webhook. 

Unfortunately I do not have a tech background, and while I believe I can create the webhook and trigger, each time I test I'm receiving either a 404 error, or a 401 error. 

My current JSON Webhook looks like this:

{
  "ticket": {
    "id": "{{ticket.id}}",
    "custom_fields": [
      {
        "id": "field.id.number",
        "value": "{{ticket.requester.custom_fields.field.id.number}}"
      }
    ]
  }
}

Could anyone offer any advice as to what is going wrong here?