A. Create a custom transaction body field.
1. Navigate to Customization > Lists, Records, & Fields > Transaction Body Fields > New.
2. Enter a Name for the custom field. (Ex. Custom Shipping Tax Code)
3. Check the Sale check box on the Applies To tab.
B. Create a workflow that will set the value of the custom field.
1. Navigate to Customization > Scripting > Workflows > New.
2. Enter a Name for the workflow.
3. Set Record Type field to Transaction.
4. Set Sub Types fields to <Transaction Type> (example Sales Order)
5. Under Initiation check the On Create and On Update check boxes.
6. Set Trigger Type field to All.
7. Click Save.
8. Click the New State button on the right section of the workflow window called Diagram.
9. Enter a Name for the State (Ex. Copy Shipping Tax Code)
11. Click the Copy Shipping Tax Code button.
12. Click the New Action button.
13. Click Set Field Value.
14. Set Trigger On field to Entry.
15. Under Parameters, set the Field field to Custom Shipping Tax Code.
16. Under Value, tick the From Field radio button.
17. Set Record field to Current Record.
18. Set Field to Shipping Tax Code.
Note: The value of the standard Shipping Tax Code field will only be copied over to the custom field if user updates an existing sales order(old sales orders). User will be able to update multiple sales orders through CSV import and the value of the standard field will be copied over to the custom field if the Run Server SuiteScript and Trigger Workflows found in Setup > Import/Export > CSV Import Preferences is checked.
Warning: Make sure to inform Administrator or the people who handles scripting customization for the NetSuite account to make sure that the workflow and the settings will not have an impact to any other customizations that are already in place. Make sure to perform testing on sandbox account if you have any.