Please note that the purpose of the RadAjaxManagerProxy control is to ease the design-time configuration only. The Proxy does not provide client-side functionality and properties as the Manager does. Therefore as the previous answers state you could set DefaultLoadingPanelID for the main RadAjaxManager, or add programmatic settings to the main Manager with RadAjaxLoadingPanel to each specific setting.
All the best,
the Telerik team
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