You can use a user control to render the desired HTML the service will return, just like this demo does: http://demos.telerik.com/aspnet-ajax/tooltip/examples/webservice/defaultcs.aspx. You can download a runnable solution with the demos site from your account if you find it easier to examine the code in Visual Studio. Of course, you can use any other approach to template the string that your service will return, depending on what you actually need.
You may also find useful this approach for defining templates to avoid any requests: http://demos.telerik.com/aspnet-ajax/tooltip/examples/databasetooltipswithoutlod/defaultcs.aspx.
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