Here a small function to mark a region on the desktop and capture that region to a bitmap handle or to the clipboard:
#include-once
#include <Clipboard.au3>
#include <GUIConstantsEx.au3>
#include <MsgBoxConstants.au3>
#include <WinAPIGdi.au3>
#include <WindowsConstants.au3>
#Region Example
;capture manual coordinates
FileDelete(@ScriptDir & "\Captured.bmp")
Global $hHBitmap = _WinAPI_MarkScreenRegionAnd