The OldMapsOnline provides a widget for partners to allow quick access to the OldMapsOnline application. The "similar maps" widget is simple code that is embeddable on other websites which creates a pretty button on the right side of page.
All you need is to add the following code into your web page which has the OldMapsOnline map identification ID:
<a href="http://www.oldmapsonline.org/link/map/cuni/1095907" id="omo_link">OldMapsOnline</a>
<script src="http://www.oldmapsonline.org/similar/cuni/1095907.js" defer></script>