How can I populate a new item using parent item properties?

I have a scenario where when a user is adding a relationship to an item I need to populate the new item with information that is dependent on the source item.


Example: A user has Part "5555" open then while in the Documents tab the user clicks the "New Relationship" button with "Create Related" selected. A new Document with Document Number set to "5555-DOC" along with other various properties of the document is then added to list of related Documents in that tab.

I've been trying a client method on the following events:

  1. onNew: I can't seem to get the context item so I don't have access to properties I need to populate the new item.
  2. afterNew on the relationship item type: creates a dangling child item I can't handle since the child seems to be created after the relationship item is created. 
  3. afterNew on the new item type: I can't seem to find the context relationship item type so I don't know if I should apply the properties or not.

New approaches or help with existing attempts are greatly appreciated. Thank you in advance!