Create a simple comboselect element

function example_elements_form() {
  $form = array();
 
  $options = array(
    'one' => t('First Option'),
    'two' => t('Second Option'),
  );
 
  $form['choice'] = array(
    '#type' => 'comboselect',    
    '#title' => t('Example'),
    '#default_options' => $options,
  );
 
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Submit'),
  );
 
  return $form;
}