Call the GetDefID member function to get the ID of the default pushbutton control for a dialog box.
DWORD GetDefID( ) const;
A 32-bit value (DWORD). If the default pushbutton has an ID value, the high-order word contains DC_HASDEFID and the low-order word contains the ID value. If the default pushbutton does not have an ID value, the return value is 0.
This is usually an OK button.