You need to get a reference of that “Tree” and connect it as object.
I do not understand what the reference should be or how to get it or whether I have to create it.
Check this documentation. It’s very similar to your case.
a reference is a specific instance of that object. so lets say you place a tree in your scene a reference would be that specific tree. if you placed two trees in your scene the reference would be the one specific tree that you want to target.