Thank you for your concern with Telerik controls.
We have updated the rendering of the advanced form and it seems that the article have become outdated. We will update it as soon as possible. Here is the code that worked correctly at my side:
sender, SchedulerFormCreatedEventArgs e)
((e.Container.Mode == SchedulerFormMode.AdvancedEdit) || (e.Container.Mode == SchedulerFormMode.AdvancedInsert))
RadTextBox attrAnnotationsTextbox = (RadTextBox)e.Container.FindControl(
Hope this will help you solve the issue.
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