$result = $client->salesOrderShipmentCreate($session, $order->getIncrementId(), array(), 'automatic send');

{
  "orderId": int,
  "createdAt": "string",
  "entityId": 0,
  "parentId": 0,
  "updatedAt": "string",
  "weight": 0,
  "qty": 0,
  "description": "string",
  "extensionAttributes": {},
  "trackNumber": "string",
  "title": "string",
  "carrierCode": "string"
}