Hello,
By default Xfce does not have a on-screen keyboard, but you can install onboard from Pamac Add/Remove Programs.
If you need it to run at the Login Screen then first you have to enable Assistive Technologies in Settings.
The you go to LightDM GTK+ Greeter Settings and in the Misc tabl you enable Keyboard under Accessibility and in the drop-down menu you select onboard
All should be set after that, and you can open onboard from menu if you need it when logged in.
To see it in Login Screen you go to the assistive icon and enable it
