Thank you for contacting Telerik Support.
I would like to clarify that we highly recommend to avoid using auto post back functionality of the RadComboBox within the advanced form. As far as I understand you want to set some additional settings for the RadComboBox such as: Filter option and set the data source and fields. Please note that the RadComboBox controls is bound to the resource data source internally and changing its data source might cause some problems.
In order to add or change some RadComboBox settings I would suggest using the advanced form as a completely separate user control. This way you have more control on the controls within the advanced form, so you can set filter options, width, height and etc. Please refer to this help resource
that provides an example of the RadScheduler will server-side and client binding. Please note that the advanced form is located in a separate user control.
Hope that this will be helpful.
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