How to use event and action properties in the tag "field"

[sqlien]

Hi,

Iwant to display à specific field when I change my drop down field to some
option; I know that it exists event and action properties , but I dont know
how to use them ?
here my code:

<field name="status"  title="Statut de la commande" >
                 <drop-down allow-empty="true">
                 <option key="Demandée" description="Demandée"/>
                 <option key="En cours ..." description="En cours ..."/>
                 <option key="Passée" description="Passée"/>
                 <option key="Planifiée" description="Planifiée"/>
                 <option key="Reçue" description="Reçue"/>
            </drop-down>
        </field>

when I change status to "Reçue" i want to display this field:

<field name="num"  title="vouchour" use-when="status==Reçue"><display/>
</field>

Thanks
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