In order to Administrate Project Zomboid with Clanforge:

Project Zomboid creates an default Admin user with a persistent password upon startup.

Username is admin.

This password can be found at:

Clanforge > Your Server > Edit Profile > Admin Password
Admin username cannot be changed via Clanforge at this time.
Admin password is set using the password you have within Clanforge upon Initial Start/World Creation.

In order to change the admin password you will need to set your admin password within Clanforge and then create a new world.