Default Assessable Value from Order Line EFF using Service Mapping and Pricing Algorithm Setup

We have defined an Order Management Extensible Flexfield (EFF) with the context code "AdhocPacking" and the attribute "AdhocPackingValue".
Our requirement is to default the Assessable Value field from AdhocPackingValue, but only when the attribute is not null.

Requesting your guidance on the recommended approach to achieve this functionality

Environment: Oracle Fusion Cloud Applications 25B (11.13.25.04.0)

We have tried Order Management Extension for the same "on Save" but the same is not working.

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import oracle.apps.scm.doo.common.extensions.ValidationException;
import oracle.apps.scm.doo.common.extensions.Message;
import java.util.logging.Level;

// Loop through all order lines
def lines = header.getAttribute("Lines");