Generating an URL for a callback

I'm porting an app to seaside that is using jTip
(http://www.codylindley.com/blogstuff/js/jtip/) to handle tooltips.
jTip works on links like the one below

<a id="element-id" class="jTip" href="/callback-url?width=450">Mouse
over for tooltip</a>

where callback-url will return the html to render as the tooltip.  How
do I generate the callback-url?

Searching the archives, the closest I could find was a snippet like:

html updater
    callback: [ :r | r render: 'bar' ];
    url.

However, this just evaluates to http://localhost:8080/seaside/myapp

Any help would be appreciated.

Thanks,
Wilkes
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