I do not understand what you are trying to say...
When you order a unit to cast an ability, it will cast the ability that corresponds to that order. You cannot create custom orders, you have to use pre-set orders which are inherited mostly from the base ability that custom abilities are based on.
If you have a custom divine shield on a unit and issue an immediate order to use the divine shield, it will use that custom one. If the unit has more than 1 divine shield ability it will pick one based on internal ordering to cast first (second will only be cast if first is on cooldown).