Thank you for writing.
RadComboBox is obsolete, you should use RadDropDownList instead. I have attached a sample project where I have tried to implement your scenario. I used the PopupOpening event to cancel the opening of the Popup instead of hiding it. Additionally, I demonstrate how you should use the SelectedItem property to select an item.
I hope this will help you. If you have further questions, I would be glad to help.
the Telerik team
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