parent - default term?

How can I do : node parent will be automatically default term? Users dont have to write the term.
I use Content Taxonomy Fields - php code for default value:

return array(
  0 => array('value' => value for value),
  // You'll usually want to stop here. Provide more values
  // if you want your 'default value' to be multi-valued:
  1 => array('value' => value for value),
  2 => ...
);