Based on the conversation through our live chat service, following feature in WebCombo has been enabled:

• multiple selection: WebCombo > MultipleSelectionSettings > Enabled="True".
• text box mode is set to ReadOnly: WebCombo > LayoutSettings > TextBoxMode="ReadOnly"

This settings populates the WebCombo in order to use checkbox mode multiple selection. There is no "search the values" in checkbox-mode multiple selection. You might want to try the other option of multiple selection such as shown in this sample. This "outlook-style" multiple selection sample allows user to search values by typing. You only need to enable MultipleSelectionSetting from WebCombo's Properties.

Hope this helps.