I solved it:
ModLoadingContext.get().registerExtensionPoint(ConfigGuiHandler.ConfigGuiFactory.class,
() -> new ConfigGuiHandler.ConfigGuiFactory(new BiFunction<Minecraft, Screen, Screen>() {
@Override
public Screen apply(Minecraft mc, Screen screen) {
return new MyConfigGuiScreen();
}
}));
(Sorry for all that needed this, I forgot to post the answer)