What do I put as the object

You need to get a reference of that “Tree” and connect it as object.

What do I connect to the ‘object’ on the cast to tree node?

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.