Hi there,
Thanks for writing in!
You cannot add a custom class to your portfolio item, it’s adding dynamically. So, if you wish to open all your portfolio items in a new window? If so, you can use following code under Custom > JavaScript in the Customizer:
(function($){
$('.entry-thumb').attr('target', '_blank');
})(jQuery);
If you wish to target individual portfolio item, you can use their unique classes (see: http://prntscr.com/6oaf3x). So, review our KB article to locate post ids (same procedure for portfolio items): https://theme.co/x/member/kb/how-to-locate-post-ids/
After that, you can use following code instead:
(function($){
$('.post-3570 .entry-thumb').attr('target', '_blank');
})(jQuery);
Replace 3570 with your post/portfolio item ID.
Hope this helps. 🙂
Thank you.