You can do this:
#include <GUIConstantsEx.au3>
#include <WindowsConstants.au3>
#include <GuiMenu.au3>
#include <GuiToolTip.au3>
#Region ### START Koda GUI section ### Form=
Global $Form1 = GUICreate("Form1", 625, 442, 370, 232)
Global $idFile = GUICtrlCreateMenu("&File")
Global $idFileExit = GUICtrlCreateMenuItem("&Exit", $idFile)
Global $idBookmarks = GUICtrlCreateMenu("&Bookmarks")
Global $idBookmark1 = GUICtrlCreateMenuItem("Bookmark1", $idBookmarks)
GUISetState(@SW_SHOW)
GUIRegisterMsg($WM_MENUSELECT, "WM_MENUSELECT")
#EndRegion ### END Koda GUI section ###
Global $g_sBookmarkTT = "This is a tooltip for bookmark1"
Global $g_hMain = _GUICtrlMenu_GetMenu($Form1)
Global $g_hBookmark1 = _GUICtrlMenu_GetItemSubMenu($g_hMain, 1)
While 1
$nMsg = GUIGetMsg()
Switch $nMsg
Case $GUI_EVENT_CLOSE
Exit
EndSwitch
WEnd
Func WM_MENUSELECT($hWnd, $iMsg, $wParam, $lParam)
Local $iIndex = BitAND($wParam, 0xFFFF)
Local $iFlag = BitShift($wParam, 16)
If BitAND($iFlag, $MF_MOUSESELECT) Then
ToolTip("")
If $lParam = $g_hBookmark1 Then
ToolTip($g_sBookmarkTT, Default, Default, "", 0, 1)
EndIf
EndIf
Return $GUI_RUNDEFMSG
EndFunc ;==>WM_MENUSELECT
Saludos