Add a custom field to a Customer

I'd like to update a customer record, adding a custom field. I'm sending the following, but getting error "Descriptor is a required field." Can someone help me see what I'm missing, please?

PUT /api/v1/base/Customer

{
  "Entity": {
    "Id": 22740,
    "CustomFields": [
      {
        "Descriptor": {
          "Id": 1099
        },
        "Value": "Door pincode 123456"
      }
    ]
  },
  "Properties": [
    "Id",
    "CustomFields.Id",
    "CustomFields.Value"
  ]
}