For networks that support SASL (such as PTirc), do the following to configure your ZNC to identify automatically on login, for each network:
/quote znc *controlpanel SetNetwork Nick $user $network your_nick
/quote znc loadmod sasl
/quote znc *sasl set your_nick nick_password
Replace your_nick with your IRC nickname on the network and nick_password with your NickServ password.
You must respect this rule as best you can if you want to keep the service.