Xpath

/control/message/message-tag

Example request

<control
    xmlns="http://webservices.ecircle-ag.com/ecm" request-id="Unique-String" group-id="1200735989">
    <message message-id="new" clone-from-id="1202515907">
        <send-date>
            <immediately/>
        </send-date>
        <message-tag>2017_campaign</message-tag>
    </message>
</control>