You can use the folloing js code:  

jQuery(document).ready(function($){
    function wpforo_topic_private_checked(){
        $("input").prop('checked', true);
    }
    wpforo_topic_private_checked();
    document.addEventListener('wpforo_topic_portable_form', function(e){
        wpforo_topic_private_checked();
    });
});

Helpful article: https://www.collectiveray.com/add-javascript-to-wordpress

This makes all topics private by default. 

The if moderators want they can make it public.