When is the best moment to set the required properties?

I tried this :

with IB_Query_Currency do
begin
IB_Connection:=DataBaseName;
KeySource :=IB_DsCustomer;
KeyLinks.clear;
KeyLinks.Add('CURRENY.ID = CUSTOMER.CURRENCY_ID');

FieldsDisplayWidth.clear;
FieldsDisplayWidth.Add('CURRENCY=30');

FieldsVisible.Clear;
FieldsVisible.add('PRAXIS_ID=NO');

SQL.clear;
SQL.add('select ID,CURRENCY from CURRENCY order by ID');

open;
end;

but the FieldDisplay settings didn't have any effects! For example, the ID
field is still visible in a LookUpCombo

Where is my mistake?

Best regards - Ulrich