Dynamic Menu - After Effects Plugin SDK

I'm working on a plugin that deals with Alembic 3D files. I can parse the geometry in the .abc and load it. What I really want is for that data to populate a dropdown menu in the plugin. But I can't seem to make the dropdown menu dynamic. So, when I select my .abc layer in After Effects comp, I can't get it to populate the dropdown with info from that file.

This reminds me of how "Visible Layers" in the Mocha plugin works. It dynamicly loads info from Mocha, but it's in a popup menu. What is that technique called?