ax - How to get ItemId by ExternalItemId for specified customer

CustVendExternalItem custVendExternalItem;
ItemId itemId;
...
itemId = "";
custVendExternalItem = CustVendExternalItem::findExternalItemId( 
  ModuleInventPurchSalesVendCustGroup::Cust, "COMPANYID", sExternalItemId );
if ( custVendExternalItem != null ) itemId = custVendExternalItem.ItemId;