$(#my_form_id).submit(function (){
   parent.window.location="/user/register";
});

$form['submit'] = array(
  '#prefix' => '<div id="super-cool-wrapper"></div>',
  '#type' => 'button',
  '#value' => 'Submit',
  '#ajax' => array(
    'callback' => 'my_super_cool_ajax_callback',
    'wrapper' => 'super-cool-wrapper',
    'method' => 'replace',
  ),
);

function my_super_cool_ajax_callback($form, $form_state) {
  // Do what you normally would with the form submission.
  return '<script type="text/javascript">
    parent.window.location="some/where";
  </script>';
}

function mymodule_showmodal($js) {
ctools_include('modal');
ctools_include('ajax');

$form_state = array(
    'ajax' => true,
  );
$commands = ctools_modal_form_wrapper('modform',$form_state);
if ($form_state['executed']) {
  $commands = array(ctools_modal_command_dismiss());
}
print ajax_render($commands);
exit();
}

function my_module_node_presave($node){
  if ($node->type == 'product'){
    $GLOBALS['redirect_parent'] = 'product_list';
  }
}

function my_module_drupal_goto_alter(){
  if (@$GLOBALS['redirect_parent']!=''){
    die('<script type="text/javascript">parent.window.location="/'.$GLOBALS['redirect_parent'].'";</script>');
    $GLOBALS['redirect_parent']='';
  }
}

<?php
echo l('Sign up here', 'user/register', array('query'=>array('destination'=>'thanks')));
?>

<?php
print l('Forgot Your Password?', 'user/password', array('query'=>array('destination'=>'thanks')));
?>