About _SendMessage, I found this perfect example of Oasis375
#include <SendMessage.au3>
#include <WindowsConstants.au3>
Run("notepad.exe")
$hWnd = WinWait("[CLASS:Notepad]", "", 10)
$hControl = ControlGetHandle($hWnd, "", "Edit1")
$text = "this is a line"
$struct_string = DllStructCreate("char[" & StringLen($text) + 1 & "]")
DllStructSetData($struct_string, 1, $text)
_SendMessageA($hControl, $WM_SETTEXT, 0, DllStructGetPtr($struct_string))