It's indeed a problem, I've checked several solutions and there is a quick one. It will work for all but the full-screen running modes. You would need to open the on-screen keyboard application (osk.exe) manually. To do it place the following code into the 'After Onset' snippet of an event with the textbox:
The folder name with 'osk.exe' is taken from my Surface Pro. If it does not exists on your device, search the folder starting with "C:\Windows\WinSxS\amd64_microsoft-windows-osk ..."', which contains osk.exe
The above code command will bring the on-screen keyboard at the top of the stimulus screen. After the text is entered, a participant can close the keyboard window.