Code:
<a href="jon.html" onclick="document.forms[0].email.value='Jon'; return false;">Jon</a>
The above assumes you only have one (1) form on the page, and the email input's name attribute is set to "email"; it also assumes that "jon.html" exists and is a form to contact Jon. This is for users without JavaScript; you may reload the same page with a query string and automatically fill-in the input based on the URI, if one is present.
PHP Code:
<input type="text" name="email" value="<?php if(isset($_GET["u"])) echo $_GET["u"]; ?>">
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