How to bind C++ property to QML property?

Perhaps an answer to a similar question would be helpful.

It shows how to connect the standard (not custom) qml property with something in C++.

To bind a property from an object you didn't create in QML (or was created in another context), you have to use Binding. In your case :

Binding {
    target: yourCppObject
    property: "cppPropertyName"
    value: yourComboBox.currentText
}