How to track anonymous user sessions with a third party app?

[hireng]

We have third party app which essentially tracks the users sessions and visitors and actions .Since the users is  anonymous user initially when the land on a store, we have this internal identifier we use to identify this anonymous user, which we update in the cart attributes, we also subscribe to webhooks like checkouts/create. 

The issue what we are facing is we are not able to process these events until we receive anonymous id in the note_attributes in the webhook payload. 

In most of the case where its a express/dynamic checkout i undertand there is a limitation of car attributes. But we noticed these are many message where we dont receive note_attributes.