There is no option to enter shared SSL URLs in HikaShop.
What you will have to do is to go in the menu Display->Views and edit the file step of the view checkout for your template and add the code below after the first <?php in order to automatically redirect to your https URL:
if(strtolower($_SERVER['HTTPS']) != "on"){
$app =& JFactory::getApplication();
$requestUri = $_SERVER['PHP_SELF'];
if (!empty($_SERVER['QUERY_STRING'])) $requestUri = rtrim($requestUri,'/').'?'.$_SERVER['QUERY_STRING'];
$app->redirect('https://my_shared_ssl_url'.$requestUri);
}
You need to replace https://my_shared_ssl_url by your shared ssl url