I advise that you use the OnClientLoad event: http://www.telerik.com/help/aspnet-ajax/editor-onclientload.html. This article also shows how to attach an event handler in the content area. I am attaching a sample and a short video that demonstrate the correct behavior.
As for DNN - we do not support this environment and the wrappers they have over our controls, so I advise that you reach out to their support team or forums for assistance with changed event handlers and property names.
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