Используй свойство ItemIndex:

Specifies the ordinal number of the selected item in the list box’s item list.
__property int ItemIndex = {read=GetItemIndex, write=SetItemIndex, nodefault};
Description
Use ItemIndex to select an item at runtime. Set the value of ItemIndex to the index of the item to be selected. The ItemIndex of the first item in the list box is 0. If no item is selected, the value is -1, which is the default value unless MultiSelect is true.