How to get "Index Price" stream via websocket?

The end point wss://nbstream.binance.com/eoptions/ws/ gives the stream for “index price” for some symbols, for example BTCUSDT@index or ETHUSDT@index. But for other coins it doesn’t work. Is there any way to have FISUSDT index price stream?

Consulting /eapi/v1/exchangeInfo, FISUSDT doesn’t seem to be a valid contract, therefore not available.

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Ok.
Is there any way to have the “Index Price” stream for Spot Margin market via websocket for other pairs like FISUSDT? I mean is there any end point for “Index Price” of margin market?
Thank you.