If you'd like to implement this using a dashlet then take a look at will's sample dashlet on github: https://github.com/share-extras/sdk-sample-dashlet
All you need to do is to modify the ftl of the dashlet:
...normal html
<a href ="http://www.mysite.com/preauth.jsp?email=${user.email}">link</a>
Another approach is to add your link to Alfresco Share's header & add the following item (http://wiki.alfresco.com/wiki/Share_Header):
<item type="external-link" id="pre-auth">http://www.mysite.com/preauth.jsp?email={useremail}</item>
{useremail} is an unkown token here, thus you'll have to customize site-webscripts\org\alfresco\components\header\header.get.html.ftl & add your token there (sorry only 4.1 code as i don't have a 4.2 in-place):
<script type="text/javascript">//<![CDATA[
var ${jsid} = new Alfresco.component.Header("${jsid}").setOptions(
{
siteId: "${page.url.templateArgs.site!""}",
siteTitle: "${siteTitle?js_string}",
minSearchTermLength: ${args.minSearchTermLength!config.scoped["Search"]["search"].getChildValue("min-search-term-length")},
tokens:
{
site: "${page.url.templateArgs.site!""}",
pageid: "${page.url.templateArgs.pageid!""}",
userid: "${user.name?js_string}",
useremail: "${user.email?js_string}",
}
}).setMessages(${messages});
Alfresco.util.createTwister.collapsed = "${collapsedTwisters?js_string}";
//]]></script>