If you want only the first name of a person set as a text of an item, you could use custom attributes
to store the person's family name. Then, on ClientSelectedIndexChanged
event of the RadComboBox, you could get the corresponding attribute for the currently selected item, and show its value in the RadTextBox control.
For more information about accessing RadComboBox item's attributes on the client, you could refer to the following help article: RadComboBoxItem Client Object
I hope this helps.
the Telerik team
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