Adding Item Line Numbers to NetSuite Purchase Order

Question

I have recently added line numbers to purchase orders in NetSuite, so that staff can refer to line numbers when talking about large purchase orders.

I used the method:

Go to Customization > Lists, Records & Fields > Transaction Column Fields > New Enter the name of the label and select the type as Free-Form or Integer Uncheck the Store Value checkbox and select applies to as PURCHASE ITEM Under Validation and Defaulting tab, check Formula and enter {linenumber} in the Default Value and click Save.

This worked well, but there are two issues with this approach, the line number is not a Store Value so can't be reported on and in "Edit" mode every line number value is "1"

Purchase order lines in view mode

Purchase order lines in edit mode

I found a similar issue in Suite Answers Answer Id: 97021 but for some reason in the workflow I cannot select my field for line number to set the secondary line number field to Store Value (it simply isn't available to select in the workflow)