Less of a TIL than a "I have to look this up every couple of years".
When you connect a Quassel Core to ZNC:
- File -> Networks -> Configure Networks...
- Manually specify...
- Network name: whatever
- Server address: localhost or your ZNC server
- Port: 2000 or what you have
- Server password:
username/network:password
Weechat for example has a dedicated password field, but here you simply have to make do with this.