hook_form_alter to create table field
you can try with hidden field. You can make #guid field a hidden field. in your hook_form_alter() #guid initially will hold the default value to empty on submission, you can make #guid fields value to whatever you want to make.

funciton mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id){
  if(isset($form_state['values']['url']){
    $guid = 'extra string' . $form_state['values']['url'];
  }
  $form['#guid'] = array(
    '#type' =&gt; 'hidden',
    '#default_value' =&gt; !empty($guid) ? $guid : '';
    );
}