1.Updated WebSocket4Net.dll to
0.15.0-beta6
2.Changed code as following
websocket = new WebSocket4Net.WebSocket(URL,
sslProtocols: SslProtocols.Tls12 | SslProtocols.Tls11 | SslProtocols.Tls,
customHeaderItems : authorizationHeaderItems);
It worked for me.