<?php bbp_forum_id(); ?> is what I’m looking for.
I added <?php bbp_forum_id(); ?> to the <body <?php body_class();?>> in the the head.php of my wp theme.
So, now, I got this <body <?php body_class();?> <?php if(is_single()) : ?>id=”parent-forum-id-<?php bbp_forum_id() ?>“<?php endif; ?>>
