/set user_name myusername
Or to connect to a specific network you have setup add /networkname for instance
/set user_name myusername/freenode
Now connect and supply the password
/connect znc.example.com 1234 password
It doesn't matter what you put for nickname as the server will readjust to what the network is set to.
Note: irssi does not like "/znc", so you will need dispatch.pl for it work. (This can be put in ~/.irssi/scripts/autorun)