You can't do this. From MSDN
If the user clicks Cancel, a zero-length string is returned.
Dim result As String = Microsoft.VisualBasic.InputBox("Enter some text")
If result.Length > 0 Then
Debug.WriteLine("Something entered and OK Clicked")
Else
Debug.WriteLine("Cancel Button Clicked OR Blank String Entered and OK Clicked")
End If
The easiest solution is just to create your own simple input form and test the DialogResult
value