Mark price

{
    "topic": "markprice@PERP_ETH_USDT",
    "ts": 1618820361552,
    "data": {
        "s": "PERP_ETH_USDT",  // symbol
        "px": "3987.2",        // mark price
        "ts": 1618820361552    // timestamp at which the mark price was calculated
    }
}