Apologies to bump, but it would be best when visiting the main page. It’s not for direct advertising - for remarketing later. For example if someone visited your forum, they are at least SOMEWHAT interested in what you have to offer, whether they register or not. That’ll allow FB pixel to send signal to remarket later.
EDIT: I placed it at admin >> customize >> css/html >> html customization >> under </head>. Is that right?