WM_GETDLGCODE

  This message is sent by the dialog box manager to a control window when a
character is pressed within the dialog box. The application should process
this message in the window procedures of any user- defined classes which it
creates. The dialog box manager needs this in order to determine certain
characteristics of a user-defined control, such as whether a user-defined
control can receive the input focus, and whether the user-defined control
"swallows" certain keys.

Parameters
  wParam and lParam are not used.

Returns
  The window proc should return one or more of the following codes:
  DLGC_DEFPUSHBUTTON -  default  push  button
  DLGC_HASSETSEL -  the  control  can  receive  EM_SETSEL  messages
  DLGC_PUSHBUTTON -  the  control  acts  like  a  push  button
  DLGC_RADIOBUTTON -  the  control  acts  like  a  radio  button
  DLGC_WANTALLKEYS -  the  control  wants  all  WM_CHAR  messages
  DLGC_WANTARROWS -  the  control  needs  the  arrow  keys  (like  a  listbox)
  DLGC_WANTCHARS -  the  control  wants  all  WM_CHAR  messages
  DLGC_WANTMESSAGE -  the  control  wants  all  messages
  DLGC_WANTTAB -  the  control  wants  the  TAB  and  BACKTAB  keys