Where do I insert it? In main, ui or in an LVGL file? It is not clear where the user can integrate his code.
See here:
ui_events.cskeletons for functions used as “Call function” event. You can add the implementation of the functions here.
ui_events.c is created when you export the UI files.
We will add some STM boards in the future, but until that you can add the UI files into an CubeIDE project created separately. You can also take look at the STM repos in the LVGL organization as a starting point.