Heine

function example_elements_form() {
  $form = array();
 
  $options = array(
    'one' => t('First Option'),
    'two' => t('Second Option'),
  );
 
  $form['test'] = array(
    '#type' => 'comboselect',    
    '#title' => t('Example'),
    '#required' => TRUE,
    '#default_options' => $options,
    '#other' => t('None of the above, but'),
  );
 
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Submit'),
  );
 
  return $form;
}