Need help with custom variables

I have Post Grids Pro and Pods on my site. I have created a custom field on my posts called flipbook_id. I am trying to create a post of my recent flipbooks on a page where if someone clicks on them it will open the flipbook in a whitebox.

I need to attach a CSS call to the feature image on the blog post with the flipbook ID number in it. Something like “open_flipbook_[flipbook_id]”. However I don’t know how to get the Wrapper custom class to include the flipbook_id.

In Post Grid I am adding a media source. The first option is the Wrapper custom class field. I can hard code the flipbook code in there and it works, but I need it to be dynamic based on the custom field I set up in the post.

My question comes down to this, how can I append a custom variable from a post inside of a Post Grid ‘Wrapper custom class’? I tried adding a short code in the field but it won’t let me do that either. Is there any way to do that?