#include <GUIConstantsEx.au3>
#include <SendMessage.au3>
#include <WinAPIMem.au3>
#include <WinAPIMisc.au3>
#include <WinAPISysWin.au3>

Global Const $WM_MYMESSAGE = _WinAPI_RegisterWindowMessage('MyMessage')

Local $hForm = GUICreate('Test ' & StringReplace(@ScriptName, '.au3', '()'), 400, 93)
Local $idInput = GUICtrlCreateInput('', 20, 20, 360, 20)
Local $idButton = GUICtrlCreateButton('Send', 165, 59, 70, 23)
GUIRegisterMsg($WM_MYMESSAGE, 'WM_MYMESSAGE')
GUISetState(@SW_SHOW)

Local $pString, $iMsg
Do
    $iMsg = GUIGetMsg()
    If $iMsg = $idButton Then
        $pString = _WinAPI_CreateString(GUICtrlRead($idInput))
        _WinAPI_SetMessageExtraInfo($pString)
        _SendMessage($hForm, $WM_MYMESSAGE, 1, 255)
        _WinAPI_FreeMemory($pString)
    EndIf
Until $iMsg = $GUI_EVENT_CLOSE

Func WM_MYMESSAGE($hWnd, $iMsg, $wParam, $lParam)
    #forceref $hWnd, $iMsg

    Local $pString = _WinAPI_GetMessageExtraInfo()

    If _WinAPI_IsMemory($pString) Then
        ConsoleWrite('WM_MYMESSAGE | WP = ' & Number($wParam) & ' | LP = ' & Number($lParam) & ' | EXTRA = "' & _WinAPI_GetString($pString) & '"' & @CRLF)
    EndIf
EndFunc   ;==>WM_MYMESSAGE