Hello Job:

I was studying this section too,  but didn't find any mechanism helping to automatically write the tracking reference.

You can see the field 'carrier_tracking_ref ' is isolated，with no method to assign value to it.  It need be input manually and just acting as an evaluation factor for generating the external tracking url(i.e. the field 'carrier_tracking_url').

At present my idea or suggestion is:

1,  interconnect with the carrier provider,  get their querying API,  use the python lib 'request' to fetch the delivery details. So far as I know, most carrier company has ready-made API for tracking service,  it's just simply http request.

2, overwrite the method 'write' to update the tracking reference or even the complete delivery details into the module 'stock.picking'.

Hope the above helps.