{{block class="MagentoFrameworkViewElementTemplate"  template="mytemplate.phtml"}}

require_once($_SERVER['DOCUMENT_ROOT'].'/app/bootstrap.php');
$bootstrap = MagentoFrameworkAppBootstrap::create(BP, $_SERVER);
$obj = $bootstrap->getObjectManager();

$state = $obj->get('MagentoFrameworkAppState');
$state->setAreaCode('frontend');

function getExistingOptions( $object_Manager,$option) {

    $eavConfig = $object_Manager->get('MagentoEavModelConfig');
    $attribute = $eavConfig->getAttribute('catalog_product', $option);
    $options = $attribute->getSource()->getAllOptions();

    $optionsExists = array();

    foreach($options as $option) {
        $optionsExists[] = ["value"=>$option['value'],"label"=>$option['label']];
    }
return $optionsExists;
}

$options=array();
$options['custom_option1']=getExistingOptions($obj,"custom_option1");