Fallout 4 Custom Menu

[Rhutos]

Hey,
I am working on remakes of my previous mods using F4SE.

I'd like to create a custom menu based on the "MessageBoxMenu" or the general pause menu.
Once I created menu's for a skyrim mod "Tame the beasts of skyrim II" based on SKI templates in the "*. fla" format.

The implementation of Custom Menus in the UI script seems very different and it's sparsely documented.
I couldn't find any uncompiled templates, so I tried to copy the "MessageBoxMenu.swf", rename it and call it via:

    Debug.Trace ("Open AVB_Test...")
    if (UI.RegisterCustomMenu ("AVB_Test", "AVB_Test", "root1.Menu_mc", new UI:MenuData))
        UI.OpenMenu ("AVB_Test")
    else
        Debug.Trace ("Menu registration did not work...")
    endif

The game pauses, until a key is pressed, but nothing shows up.
If I try a different "root path" the game freezes.

All "Modding resources" I found, are meant for interface mods/add-ons, not for interactive menu's.

What I am missing?