add new network

/network list
/network add LiberaChat
/server add -auto -network LiberaChat irc.libera.chat
/save

sasl login

/network add -sasl_username <login> -sasl_password <password> -sasl_mechanism PLAIN LiberaChat
/server add -auto -net LiberaChat -tls -tls_verify irc.libera.chat 6697
/save

join and leave channels

/join #nim /part #nim

auto join channels

/channel add -auto #gemini LiberaChat

browse popular channels

https://netsplit.de/channels/?net=libera.chat

hide login/logout messages

/ignore * JOINS PARTS QUITS

/network add -sasl_username k0ta -sasl_password TX5awf1Dfj5b5ilz9VxGWPavl -sasl_mechanism PLAIN TildeChat

script command missing

First install the irssi-perl package if you haven't. Then make the /load perl command autorun:
echo 'load perl' >> ~/.irssi/startup

scriptassist error

You probably need this perl package perl-LWP-Protocol-https