For example, I have an object layer for creatures and all the objects in that layer need custom properties “ClassName” and “PackageName”. The properties don’t have to be set, but I want them to be there so whoever works with the map, knows those properties need to be filled in.
Also, is there a way to copy paste an object in a object layer?
Not at the moment. Objects can inherit their type only from their tile. That means, that if your creatures are placed as tiles, setting a “Creature” type on those tiles, and specifying the “ClassName” and “PackageName” properties for that type in the Object Types Editor, will mean that any placed creature shows those properties.
Maybe it makes sense to consider inheriting the object type also from its layer, or to be able to set a default initial type for newly created objects. Or, this could be something to enable by using a scripting hook.
Normally simply selecting it and pressing Ctrl+C and Ctrl+V works. Are you facing a problem with it?