There can be cases where you need to define your own intializeEngine, to e.g. expose an enum to JS:

https://doc-snapshots.qt.io/qt6-dev/qtquick-how-tos.html#use-a-c-enum-in-javascript

https://doc.qt.io/qt-6/qtqml-writing-a-module.html almost shows how to do this in CMake, but:

The plugin and the backing library both are in the same CMake target imageproviderplugin

For the scenario where they are separate targets, the user needs to do something like this:

https://code.qt.io/cgit/qt/qtdeclarative.git/tree/src/quick/CMakeLists.txt#n257

Except they'll need to use target_sources to add the plugin's .cpp file to the plugin target:

target_sources(MyModuleplugin PUBLIC
    Plugin.cpp
)