Hey @Dunhou,
I have edited your posting. It is fine to mention the beta forum, but we would rather not see its name written here.
When you are talking about 'C++', I assume you are talking here about the Windows SDK/API, as for example GetGUIThreadInfo. I would doubt that you get very far with that here. Cinema 4D is very OS agnostic and we have our whole GUI decoupled from the OS. The little rename window in our tree view control is not an OS rename dialog. I am not even sure if this is something with an HWND handle which you could address so that you could intercept keyboard events to it, or if this is just a virtual window within our API.
For this IMM thing to work, we would have to use the IME API in Cinema 4D, so that our text edit gadgets support it. I also do not quite understand what you are trying to do. The missing piece is here probably that our text edit gadget does not send its current content to the IME API (at a glance, via ImmSetCompositionStringW as shown here). And as a third party, you cannot really fix that. Because you (a) do not own the implementation and you (b) are not informed about an edit event, so you cannot just write a plugin which sends the object name to the IME API.
Cheers,
Ferdinand