I asked that question back in July on the MSDN forums. The answer that I got was:
"This has been answered here:
https://social.msdn.microsoft.com/Forums/en-US/d81ebfa0-ccdc-479e-8e2f-bc921de17343/onscreen-keyboard?forum=WindowsIoT
There's no onscreen keyboard available at the moment. There's the possibility that one will be added later, but it's not guaranteed. If you read the linked post you also find an explanation as to why none is available."
Just for reference, here's the link to the MSDN question that I asked: https://social.msdn.microsoft.com/Forums/en-US/9c95f424-f0ca-4751-9497-a019c1d40907/touch-keyboard?forum=WindowsIoT
And here's the answer from the linked post:
"At the moment there is no On-Screen Keyboard available. If you want a user to input something via the screen then you’ll want to provide the UI for that input in your application. Part of the reason for this is that we don’t have a ‘shell’ like explorer that is there helping control what app is running or providing other services such as SIP / OSK as is handled on the desktop.
Longer term I believe this is something that the product team is considering / looking into but I do not have any timeline on if or when it might be an option (if ever). If I learn otherwise I will let you know.
I hope that helps."