How to use custom IV avatars using API?

HI, I want to create a video with my custom avatar (using Avatar IV Digital Twin engine, not regular) via API. I see that there is a separate endpoint for Avatar IV videos, but it doesn't have an avatar ID property, so apparently I can only create a new avatar from a photo I upload. But i want to use a custom avatar.

How do I do that? Is there a way to use custom Avatar IV Digital Twins via API directly? Is it possible to do that using a regular V2 endpoint?

Thanks!