New to IPS..

What's the best way to get parent_id when landing on a specific forum (+ sub-forums and it's topics) using global_template?

Basically, I'm hard coding a menu item and I want to set it active when browsing within it's pages.

I know I can hardcode the forumids but some categories just have too many sub-forums. 

Thanks.

It's depending on the site.
If you really mean the forum like https://invisioncommunity.com/forums/forum/505-product-guidance/ you can get it via \IPS\forums\Forum::load( \IPS\Request::i()->id )->parent()->_id ; 
If you need it when you're inside a topic, you can access it via \IPS\forums\Topic::load( \IPS\Request::i()->id )->container()->_id ;

So you'll need a construction which determines if you're viewing a forum or a topic and depending on this load the forum direct or the topic and then grab the forum data