The solution here is, in the URL you have to initialize the "inventorylocation" e.g. https://{NS_ACCOUNT}.suitetalk.api.netsuite.com/services/rest/record/v1/salesOrder/146401/!transform/itemFulfillment?init=inventorylocation:21

then in the body you have to pass the JSON in the below format,

{
"item": {
    "items": [
        {
            "orderLine": 1,
            "quantity": 1.0,
            "location": 21,
            "itemreceive": true
        },
        {
            "orderLine": 2,
            "quantity": 1.0,
            "location": 21,
            "itemreceive": true,
            "inventoryDetail": {
                "location": 21,
                "quantity": 1.0,
                "inventoryAssignment": {
                    "items": [
                      {
                        "binNumber": {
                            "id": 21
                        },
                        "inventoryStatus": {
                            "id": 1
                        },
                        "issueInventoryNumber" : {
                            "refName": "80130SUBD0083", // Text
                             "id": 82 // value
                        }
                    }
                  ]
               }
            }
        }
    ]
}

}