Get extra fields in REST API

After a customer places their order, extra fields are saved. You can get them later via REST API in the extraFields and orderExtraFields fields of an order. See Orders in REST API

// Request

GET /api/v3/4870020/orders/20 HTTP/1.1
Content-Type: application/json;charset=utf-8
Authorization: Bearer e***s0
Cache-Control: no-cache

// Response

    "total": 12.35,
    "orderNumber": 104,
    "extraFields": {
        "reference_number": "#334-3340-1"
    "orderExtraFields": [{
        "id": "reference_number",
        "value": "#334-3340-1",
        "customerInputType": "TEXT",
        "title": "Affiliate numer",
        "orderDetailsDisplaySection": "billing_info",
        "orderBy": "1",
        "showInNotifications": true,
        "showInInvoice": true