ObjectGraphBuilder select specific subclass for a property

Question

I have the following class hierarchy:

class Tree {
    Branch branch;
}

abstract class Branch {
    int length;
}

class SubBranch1 extends Branch {}

class SubBranch2 extends Branch {}

I'm trying to use ObjectGraphBuilder to instantiate a Tree as follows:

builder.Tree {
   branch(length: 3)
}

How do I tell ObjectGraphBuilder which Branch subclass to use?