The mentioned value was available in the old classic version of the RadAjaxManager control. Currently it is not applicable in the radControls for ASP .Net Ajax.
You could use the PageRequestManager Initialize event to re-write the url during Ajax request. For example:
form = Sys.WebForms.PageRequestManager.getInstance()._form;
form._initialAction = form.action = window.location.href;
I hope this helps.
the Telerik team
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