The new “add owner to opportunity” feature allows you to dynamically add an owner to an opportunity. However, to do this, you must use the userID; it does not work with the full name. But in the available variables: in the appointment ➔ assigned user category, you cannot retrieve the userID. So, you either have to add the userID in appointment ➔ assigned user, or you have to be able to choose the owner using the full name.