Yes, Kevin is right about the empty text and the empty message check.
If you want to save the text typed in the input field, in a database, you could, before a post-back to the server is made, check if an item with such a text already exist in the items collection. To achieve this, you could use the FindItemByText(text)
method of the RadComboBox's client object.
For more information about the client-side API of the RadComboBox, you could refer to the following help article: RadComboBox: Client-side API
I hope this helps.
the Telerik team
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