The idea is to save in a session variable the original discussion and then on the qListForm to consume the values from the session.
Set the session on the qDiscussionView, then on the qListForm go to the Display Fields, pick the Email To field, hit Edit then scroll all the way to the bottom and then pick the field type to be from the session.
When you reply to a discussion you pick the discussion and when you Reply it will set the session values, open the qListForm and it populates the field with the value from the session.
Please check the docs about how to use the session.