Please check the Automatic Load On Demand example again - Filter property is set in the code-behind in Page_Load event according to the checkboxes in the configuration panel:
sender, EventArgs e)
RadComboBox1.Filter = FilterStartsWith.Checked ? RadComboBoxFilter.StartsWith : RadComboBoxFilter.Contains;
RadComboBox2.Filter = FilterStartsWith.Checked ? RadComboBoxFilter.StartsWith : RadComboBoxFilter.Contains;
It will work in the same way in a combobox populated from WCF Web Service.
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