[@robin-w Moderator 11 months, 2 weeks ago]

I’m pretty sure that the buddypress function hooks to the new_forum_handler in bbpress, so this should work

add_action( 'bbp_new_forum_post_extras', 'rew_add_pg', 10 , 1) ;

function rew_add_pg ($forum_id ) {
	 add_post_meta( $forum_id, '_private_group', 'group1', false );
}

Let me know of not.