Create a single client broker. Set org_group if your organization uses org groups.
Unlike /external/client-accounts and /external/client-underwriters, this resource has no unique constraint on (org_group, sub_org_group, client_broker_id), so a POST with a duplicate natural key inserts a second row rather than returning 409. To upsert by natural key, use POST /external/client-brokers/bulk with a single item, or PATCH /external/client-brokers/{id}.