A common ComboBox control is like this:
code:
<Control xsi:type="ComboBoxControl" Id="OptionName">
<Position Top="10" Width="100" Left="10"/>
<Items>
<Item Id="ItemId1">Name of the first item</Item>
<Item Id="ItemId2">Name of the second item</Item>
</Items>
<Help>This text appears when you hover your mouse over the box. It is usually a help text.</Help>
</Control>