Not a REST call: this is the WebSocket handshake. Connect to the URL below and the exchange answers 101 Switching Protocols; everything after that is frames, not requests.

Public: no key, no signature.

Full-replace futures order-book snapshots for the pairs you name.

{ "Method": "subscribe", "CurrencyPairs": ["BTCUSDT"] }

{
  "currencyPairCode": "BTCUSDT",
  "asks": [{ "price": 64181.0, "quantity": 0.7 }],
  "bids": [{ "price": 64180.5, "quantity": 1.2 }]
}

This is the same shape as GET /api/futures/trade/orderbook/{code}. Each frame replaces the whole client book, contains at most 20 levels per side, and is pushed at most every 500 ms per pair. Futures has no delta channel: re-read this snapshot each time. Pair codes have no underscore; /ping is shared.