Setting tax code for sales order item through Rest API

We are using the REST API in order to create sales orders in NetSuite from an external system.

Everything works fine except when trying to set the tax code for the items. It simply does not work. The sales order is successfully created but the specified tax code is not applied to the items (and no error code is returned). 

This is a sample of the payload we are sending to the REST API endpoint:

{

 "entity": { "id": 1529811 },

 "salesRep": 41,

 "subsidiary": 7,

 "memo": "TEST TAX CODE 5",

 "currency": 4,

 "item": {

  "items": [

   {

    "item": { "id": 139, "taxcode" : { "id": 933 } }

   },

   {

    "item": { "id": 1116, "taxcode" : { "id": 933 } }

   }

  ]

 }

}

I found an article in Internet where a person said that he was told by the NetSuite support team that setting the tax code for an item is only allowed when SuiteTax is enabled. Is that correct? Is this what is causing the tax code not to be set in our case (we are using the legacy tax calculation option)?