gtk:window-default-widget (window)

window -- a gtk:window widget
widget -- a gtk:widget object to be the default, or nil to unset the default widget for the toplevel

Gets or sets the default widget for the window. The default widget is the widget that is activated when the user presses the Enter key in a dialog for example. When setting, rather than unsetting, the default widget it is generally easier to call the gtk:widget-grab-focus function on the widget. Before making a widget the default widget, you must call the gtk:widget-can-default function on the widget you would like to make the default. The C library has the gtk_window_set_default() function, which is implemented as the (setf gtk:window-default-widget) function in the Lisp API.