Reply to myself…
After invoking (using browser):
http://192.168.xxx.yyy/rpc/Ws.SetConfig?config={“enable”:true, “ssl_ca”:"*", “server”:“ws://192.168.xxx.zzz:8123/api/shelly/ws”}
The option to configure “Outboud websocket” became visible on network configuration.
192.168.xxx.yyy : address of Shelly Plus HT
192.168.xxx.zzz : address of HA
You can also get current websocket configuration: