yes, converting the code to create an array of the coupon codes will cause the server behavior to not show in the server behaviors panel
The idea is that by adding [] to the end of the session variables name, it is now creating an array.
The custom trigger code to use would be:
isset($_SESSION["PromoCode"]) && in_array("12345",$_SESSION["PromoCode"])
This custom trigger is a replacement of the trigger that is used in the tutorial.