Answer
Populating a Client_SYS attribute string alone doesn't create anything — the code is missing the actual call to the target LU's own New method (in this case, Inquiry_Supplier_API.New(attr_)) that takes the populated attributes and creates the record. Also make sure the attribute string includes every key the target record needs, not just the value you're trying to set — for a Request for Order Quotation Supplier record, that means adding both the Inquiry Number (the parent key) and the Vendor Number to attr_ via Client_SYS.add_to_attr before calling New, since a record can't be created with only one of its two keys populated.