You have to edit the overlay.xml file:
http://www.radiantviewer.com/dicom-viewer-manual/overlay_xml.htm
Add your custom annotation row:
<tl>
<i r="0" s="1" c="1"/>
<i r="1" s="2"/>
<i r="2" s="3"/>
<i r="3"><e k="1" i="00181400"/></i>
</tl>
And you should be able to view the 0018,1400 tag value in the upper left corner.