You can select an item in several ways:
If you are initializing items declaratively inside your web page, you can set the selected attribute to the desired List Item element.
By setting the selectedIndexes or selectedValues properties to the correposnding value.
By calling the select method. This method accepts one argument – a List Item element instnace. You can get a reference to it by calling the getter items on the Combo Box element and filter out the item you desire.