how to default a field based on other field in redwood using visual builder studio??

Summary: Defaulting Field value based on Another Field.

Content (please ensure you mask any confidential information):

We have a Requirement to Auto populate the Business unit field based on the selected Legal Employer Field.

Eg: In below snapshot , I have selected legal employer as XXXXXXX

The Business Unit field needs to be Auto populated with the same .

I have tried using Default value option using Business Rule using Expression, Please find the Snapshot and Field selected for reference.

[[ $fields.employmentAssignments.whenAndWhy.LegalEntityId.$numberValue() ]]

But this not working as expected, can anyone suggest.

Version (include the version you are using, if applicable):