How to set field value on custom segment and make it mandatory?

Hello!

We have an instance of Netsuite that houses two separate companies (under one parent company). We have subsidiaries set up but our integrations team needs an indicator that will live on all records to easily be able to distinguish which of the companies it belongs to and they do not want to use the subsididary field since we have multiple for each company. This includes items, transactions, entities, etc.

I thought a custom segment would be best for this since it seems you can apply it to any type of record but after creating, I'm unable to get the field to show on our custom Item forms. It's nowhere to be found in the customization subtabs. I need this on the main body of the form, not a sublist. On the custom segment itself, it is applied to pretty much every record/form type so I'm not sure why it's not showing as an option on the customize page.