is_default = true-false; 

Specifies whether the button is the default button selected (“pushed”) when the user presses the accept key. Possible values are true or false (default: false). If the user is in an edit_box, list_box, or image_button that has the allow_accept attribute set to true, the default button is also selected if the user presses the accept key or (for list boxes and image buttons) double-clicks. The default button is not selected by the accept key if another button has focus. In this case, the button that has focus is the one selected.

Only one button in a dialog box can have the is_default attribute set to true.