I can only assume that, that might depend on the game's ability to make API calls to the host OS to evoke the On Screen Keyboard (osk.exe)
Generally you can type osk.exe from any number of places from within the OS or you can go here C:\Windows\System32\osk.exe
and pin to start, pin to taskbar, or send to desktop (shortcut). Whether any of that will actually help or not is going to depend on what you are able to do from the game when it is prompting for a username. Usually touch enabled games when you touch and hold the area where you are expected to enter keyboard characters, will evoke the On Screen Keyboard.