$form['add_items']['add'] = array(
        '#type' => 'submit',
        '#name' => 'add',
        '#value' => t('Add'),
    '#ajax' => array(
    'callback' => 'add_items',
    'event' => 'keyup',
    'progress' => array(
            'type' => 'throbber',
            'message' => NULL,
    ),
    )
    );

    $form['items'] = [
        '#type' => 'table',
        '#header' => [
            t('Item')
        ]
    ];
 }

$form_state->setRebuild(true);

return $form['items'];