#include <atlddx.h>

class CMainDlg : public CDialogImpl<CMainDlg>, public CUpdateUI<CMainDlg>,
    public CMessageFilter, public CIdleHandler,
    public CWinDataExchange<CMainDlg>
{
    CComboBox m_wndCombo1;
public:
  ...
    
    BEGIN_DDX_MAP(CMainDlg)
        DDX_CONTROL_HANDLE(IDC_COMBO1, m_wndCombo1)
    END_DDX_MAP()

LRESULT CMainDlg::OnInitDialog(UINT /*uMsg*/, WPARAM /*wParam*/, LPARAM /*lParam*/, BOOL& /*bHandled*/)
{
  DoDataExchange(DDX_LOAD);

  ....
    
  m_wndCombo1.AddString("bla-bla-bla");

  return TRUE;
}