Access Window's Item ID in Item View Relationship Command Bar Default Button CUI Click Method context

For contextual information, the tabs in which we have for say viewing a specific Item, these tabs themselves have a command bar, denoted as Relationship Command Bar due to their location being within another item.

With the CUI method of appending a new item here, the options context is the Item of type=RelationshipType. Options and Options.itemTypeName is only about the type="RelationshipType".

Is there a way to pass context of the item that I am viewing? It seems as if I'll need to use the 'Additional Data' field within the CUI button. Advice on passing item data with that or with another solution? The traditional accessors are only about the relationship type

Thanks,

Frank