Issue adding PO with API

I have a task to upload multiple POs into FreshService. I am able to post a test PO using the code below, as long as the ‘created_at’ field is not present. However, I need to include the date when the POs were placed, but I am encountering an error (listed below). How can I use the API to publish POs while also including the ‘created_at’ date for when the orders were placed?

{
    "vendor_id": 31000414470,
    "name": "Carl API test",
    "po_number": "PO-18",
    "vendor_details": "AMAZON MKTPLACE",
    "expected_delivery_date": "2024-09-14",
    "created_at": "2021-02-11",
    "shipping_address": "123 main street",
    "billing_same_as_shipping": true,
    "billing_address": "123 main street",
    "currency_code": "USD",
    "discount_percentage": 0,
    "tax_percentage": 0,
    "shipping_cost": 0,
    "purchase_items": [
        {
            "item_name": "Pizza",
            "description": "Lunch",
            "cost": 20,
            "quantity": 1,
            "tax_percentage": 8,
            "item_type": 3,
            "item_id": 31000036332
        }
    ],
    "workspace_id": 2
}

Error

    "description": "Validation failed",
    "errors": [
        {
            "field": "created_at",
            "message": "Unexpected/invalid field in request",
            "code": "invalid_field"
        }
    ]
}