Error generated when saving a new order management via script

Summary

When entering a script that lets the system add a value to the the field ShipSetName and saving the record an error is generated

Content

Hello all,

Using the task Manage Order Management Extensions task I entered the below script:

setAttribute("ShipSetName", ItemNumber);

When I create a new order and pressing save, the following error is generated:

An error occurred when running extension Order_Management, during event On Save: oracle.jbo.JboException: JBO-29000: Unexpected exception caught: groovy.lang.MissingPropertyException, msg=No such property: ItemNumber for class: ExID300000021003189__3. (DOO-2685874)

I am going through the scripts found in Doc ID 2428750.1 but since I am new to scripting I have little knowledge of the content since my knowledge in scripting is basic. Is my script entered above correct?