How to stop Hiring Manager field defaulting to logged in user?

Summary:

We have managed to set the Hiring Manager field in job requisitions to default to the logged in user so it autofills for the Hiring Manager completing the job requsition. We need to set a business rule to allow this when blank but not default to the Recruiter when they open the posting to review. Has anyone successfully managed to set this up?

We used this expression to default the Hiring Manager field

[[ $fields.selfEmploymentDetails.AssignmentId.$value().toString() ]]

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