An application sends a DM_GETDEFID message to retrieve the identifier of the default push button control for a dialog box.
DM_GETDEFID
This message has no parameters.
If a default push button exists, the high-order word of the return value contains the value DC_HASDEFID and the low-order word contains the control identifier. Otherwise, the return value is zero.
The DefDlgProc function processes this message.