Scripting within a area in the game


I need to have a area within the game in which things will randomly instantiate. Is this possible to achieve in the editor? For example to draw some boundaries in an area and apply a script there. Or do I have to manually hardcode the area boundaries I want stuff to instantiate in?

EDIT: I have a terrain in which I want an area that will randomly spawn stuff


Do it manually. Just put a trigger in the area and some empty game objects in it. When the player enters that area then instantiate the objects in the position of the game objects that you created. Generate some random numbers to decide which positions the objects will instantiated.