Discount codes in Express Checkout

{
    "profile_id": "default",
    "url": {
        "return_url": "https://merchant/return"
    },
    "order": {
        "items": [
            {
                "amount": 2009990,
                "vat_amount": 401998,
                "vat": 25,
                "quantity": 1,
                "line_id": "l1"
            }
        ],
        "amount": 2009990,
        "vat_amount": 401998,
        "currency": "NOK",
        "merchant_reference": "reference"
    },
    "express": {
        "shipping_options": [],
        "discount_codes": {
            "callback_url": "http://merchant/discount_code_callback_url",
            "max_count": 3
        }
    },
    "configuration": {
        "discounts": {
            "express_discount_codes": {
                "enabled": true
            }
        }
    }
}