global $user;
 
$comment = array(
  'nid' => 123,
  'pid' => 0,
  'uid' => $user->uid,
  'name' => $user->name,
  'author' => $user->name,
  'subject' => 'Comment subject',
  'comment' => 'Comment body',
);
$form_state = array();
$form_state['values'] = $comment;
$form_state['values']['op'] = t('Save');
 
drupal_execute('comment_form', $form_state, $comment);

Drupal 6