Set up the "keyboard" with a letter on each and SPACE on the space bar.

ALL of the keys should run this code.

Sub AddText(oshp As Shape)
Dim strKey As String
Dim osld As Slide
strKey = oshp.TextFrame.TextRange
If strKey = "SPACE" Then strKey = " "
Set osld = SlideShowWindows(1).View.Slide
With osld.Shapes("Title 1")  'change the name
.TextFrame.TextRange = .TextFrame.TextRange & strKey
End With
End Sub
keys.pptm

NOTE it will only work in show mode and you might have to change the name of the shape to show the text.