MACROMEDIA FLASH MX 2004-ACTIONSCRIPT LANGUAGE Reference page 725

status_txt.text = "fill in username";
Selection.setFocus("username_txt");
return false;
}
if (password_txt.text.length == 0) {
status_txt.text = "fill in password";
Selection.setFocus("password_txt");
return false;
}
status_txt.text = "success!";
Selection.setFocus(null);
return true;
}
See also
Selection.getFocus(), Selection.onSetFocus
Selection.setFocus() 725