I found this function:
Func WinGetTrans($sTitle, $sText = "")
Local $hWnd = WinGetHandle($sTitle, $sText)
If Not $hWnd Then Return -1
Local $aRet = DllCall("user32.dll", "int", "GetLayeredWindowAttributes", "hwnd", $hWnd, "ptr", 0, "int_ptr", 0, "ptr", 0)
If @error Or Not $aRet[0] Then Return -1
Return $aRet[3]
EndFunc
and used it:
$wgt = WinGetTitle("")
WinSetTrans($wgt,"",100)
MsgBox(0,"",WinGetTrans($wgt))
WinSetTrans($wgt,"",255)
Func WinGetTrans($sTitle, $sText =