Currently we can not provide you with a suitable workaround to this issue, but I would suggest you to use the filtering feature of the ComboBox combined with the OpenDrowDownOnFocus
properties. Here is a sample code snippet representing the described scenario:
This will make RadComboBox control to open its drop down menu and filter the displayed items corresponding to the entered text.
More information on filtering in RadComboBox here
All the best,
the Telerik team