If you want the opening only, set the order type to OPG. That should be all you need. This order type may not be available by default, in which case you have to enable it through "Global Configuration.
If you want a limit order that will participate in the open, set the exchange to SUPERSOES and disable "Regular trading hours only" in the time in force panel.
In either case the order must be a round lot. SUPERSOES does not take odd lot orders.
Martin