Allow _ispressed() but block input

#include 

HotKeySet("{ESC}", "_Quit")

While 1
If _IsPressed('41') Then
MsgBox(0, 'Test', '')
EndIf
WEnd

Func _Quit()
Exit
EndFunc ;==>_Quit

Hello, how do I modify the script without using hotkeyset() such that pressing 'a' would only result in the display of the message box and wouldn't send 'a' to the active window (e.g., would not type 'a' on an active word processor window)?

Thank you!