You can't add a link to a rectangle element in the landing page editor, but you can create a more flexible HTML element that has a clickable area (parts borrowed from Stackoverflow):
<a href="http://www.google.com"> <!-- Link tag wraps the entire div/rectangle -->
<div style="background-color:green; padding:15px;"> <!-- the div is your rectangle -->
<span style="position:absolute; width:100%; height:100%; top:0; left: 0; z-index: 1; background-image:url('spacer.gif');"></span> <!-- the span tag makes the area clickable -->
<p style="position:relative; z-index:9999"><a href="http://www.yahoo.com">Different Link in the box</a></p>
</div>
</a>
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