Proxy to backend for WebSocket

It's perfectly working for Http, but not for WebSocket. I did not find the documentation about this. How can I do proxy for WS?
ng serve --proxy-config proxy.conf.json
proxy.conf.json:

{
 "/": {
 "target": "http://localhost:8080",
 "secure": false
 }
}

{
 "/": {
 "target": "http://localhost:8080",
 "secure": false
 "ws":true
 }
}

[lidebug]

{
 "/ws": {
 "target": "ws://localhost:8000",
 "secure": false,
 "ws": true
 }
}

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