Going by the documentation I think you're going to have to use a For Loop searching for the string you're looking for which returns the position of the iterator.
var idx: int = GetPositionInItemList(searchValue, itemList)
func GetPositionInItemList(searchVal: string, _itemList: ItemList) -> int:
for i in _itemList.get_item_count():
if _itemList.get_item_text(i) == searchVal: