Is it possible to change the Fedex reference number on item fulfillment packages automatically?

I’m filing this under SuiteScript, but I’m starting to suspect it may not be possible within SuiteScript… but here goes:

I’m trying to create an automation that will automatically add data to the  reference1FedEx field when an item fulfillment is created. The intention is to get that information onto the shipping label, for visibility for the warehouse team.

The problem is, I have no idea how to write any values to that field. The packages sublist in the records browser doesn’t mention that field, although it is visible here on the schema browser: https://www.netsuite.com/help/helpcenter/en_US/srbrowser/Browser2018_1/schema/other/itemfulfillmentpackagefedex.html?mode=package

In trying to hack it, I’ve realized that I don’t seem to be able to set any package sublist fields — it just returns an unexpected error. If I do getLineCount on the package sublist, I get -1, which I think indicates that the packages haven’t been “created” yet even though they were created in the UI?

Is there some step I need to take to make the package fields writeable? And if I get past that, will I be able to set the reference field, or is that going to just end in failure anyway?

Any other ideas for a way to tackle this? I’m stumped.

UPDATE: Worth noting, we have fedex label integration set up, and have pick/pack/ship turned off. UE Script is currently running on create, beforesSubmit.