If you decided to design your WEBKiosk with a login option, you need to activate it in your WEBKiosk settings (possible with the PROKiosk plan).
Go to "My WEBKiosk" --> "Tasks" --> "Edit Design".
Then click on the little pen icon in the white box of the navigation widget:
Under "Links" and the "URL field" you can choose the option "#login".
Here you learn more about the Login Button at WEBKiosk.