When previewing an order, all order validation rules are run, including buying power checks. All applicable fees and commissions are delivered in the response. Submit the order with all necessary parameters and include the additional parameter of preview=true

Response

{
  "order": {
    "status": "ok",
    "commission": 3.49000000,
    "cost": 34.715100000000,
    "fees": 0,
    "symbol": "T",
    "quantity": 1,
    "side": "buy",
    "type": "market",
    "duration": "day",
    "result": true,
    "order_cost": 31.225100000000,
    "margin_change": 0,
    "request_date": "2019-05-14T15:56:47.371",
    "extended_hours": false,
    "class": "equity",
    "strategy": "equity",
    "day_trades": 3
  }
}