When I create a new object and without saving I want to create a child, I can't do it because the parent doesn't exist yet. How can I automatically save and substitute the parent when creating a child element?

I tried to do it by onchange method, it works, so I can't create and save parent_id in child one, but in this case problem is that frontend doesn't know about that and when user want to save parent, he can't to do it, because this parent was created on backend side.

I wanted to attach a video, but karma won't let me do it.
https://streamable.com/xoyf0s