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>