add_filter('woocommerce_checkout_fields','new_select_field_classic_checkout_options');

function new_select_field_classic_checkout_options($fields) {
    $fields['billing']['referral_info'] = array(
        'type' => 'select',
        'label' => 'Who referred the store?',
        'priority' => 121,
        'required' => false,
        'class' => array('form_row_wide', 'bg-main'),
        'validate' => array('tel'),
        'autocomplete' => 'tel',
        'placeholder' => 'Shipping phone (if different than in billing)',
        'label_class' => array('bg-cta'),
        'clear' => true,
        'options' => array(
            'search' => 'I found it through search results',
            'affiliate_link' => 'I came from an affiliate blog',
            'video' => 'I found the link in the video description',
            'ad' => 'I clicked an ad',
        ),
    );
    return $fields;
}