Hi Ben,
In type & attribute manager, add an attribute to the CAD type as normal. Once added (and still in edit mode) select the newly added attribute and in the top-right window, search for the application you want to connect with (in this case Autodesk Inventor). In the field next to the application's label, enter the name of the attribute as defined in the application.
After a "save" and "done you're set to go