You can use the OnClientPageLoad
. Now, in case you are referring to the server-side Page_Load event of the page loaded in the right-hand side RadWindow - this is not possible. You cannot access different pages in the code-behind. You could create a global class to store needed data and then initiate an AJAX request to reload your UC and load the data from this class for example, yet cross-page communication is a general task that is not related to the RadControls. You could also inject a script that will invoke a function from this page, obtain needed data, call a function on the parent page (where the splitter is) that will, again, access the left pane as described above.
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