How to set rules for default sort in TreeTableView in JavaFX?

Question

I searched on SO, but only found posts regarding sorting TreeView, e.g., JavaFX: TableView: Arrow for a column sorted by default and Javafx: Re-sorting a column in a TableView.

Both don't seem to work with my TreeTableView.

Also Oracle's blog post on Tree Table Views (see Example 15-4: Sorting Mode Settings) does not really help.

I think it should be some combination of

row.setSortType(...)

and

table.setSortMode(...)

and

table.sort()

and

table.getSortOrder(). ...

and

table.getSortPolicy(). ...

yet I could not get it working.

What I want to achive:

1. Define columns & add them to the table (Works fine)
2. Set default sort (e.g., sort data by "valueColumn" descending) (How?)
3. Set root (Works, but does not get sorted at the moment)