Or you can contribute to nemo-dbus plugin

Agree that this would be the best thing to do. Unfortunately that is not possible for me at the moment as all my development is done on the phone. It's why I am using qml and python instead of c++.

For those that are interested, this works using dbus python. It requires dbus-python3 package to be installed. Here is the python code:

import dbus
def setFavorite():

bus = dbus.SessionBus()
ambD = bus.get_object('com.jolla.ambienced','/com/jolla/ambienced')
ind = ambD.createAmbience('file:///usr/share/ambience/harmony/harmony.ambience',dbus_interface='com.jolla.ambien ced')
a_dict = {"favorite": True}
works = ambD.saveAttributes(1, ind, a_dict, dbus_interface='com.jolla.ambienced')