You can fill combo box two way first you can use lookup method using extended data type second if you are not using EDT then you can fill combo box by using following code.
public void fillCombo()
{
CustTable custTable2;
ComboBox1.clear();
SaleCalls.ParentOffice = "Abc";
custTable2 = CustTable::find(saleCalls.ParentOffice,false)
rec = custTable2.RecId;
while select * from Address where Address.AddrRecId == custTable2.RecId
&& Address.AddrTableId == custTable2.TableId
{
// ComboBox1.add(int2str(Address.RecId));
ComboBox1.add(Address.Name);
}
ComboBox1.selection(0);
}