bbPress.org

Why not add the below code to the theme’s functions.php file?

//Subscribe by default
function subscribed_by_default(){
echo '<script type="text/javascript">jQuery("#bbp_topic_subscription").prop("checked","checked");</script>';
}
add_filter('bbp_theme_after_topic_form_subscriptions','subscribed_by_default');
add_filter('bbp_theme_after_reply_form_subscription','subscribed_by_default');