How can you autopopulate a work order attribute with the Inventory Category of the Primary Item?

Summary:

We need to use the Inventory Category of the Primary Item of Work Orders in an SLA to derive an accounting segment for a Maintenance Material Issue. We haven't been able to find this as a source in the rules so we are investigating populating the value in Work Order Attribute Number 2, which can be used as a source. We do not want users to manually populate this value when creating the work order. How can we autopopulate the field with the information (which is easily attainable by looking at the primary item)?

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