If this is actually a page, then you can set the selected navigation tab in the page's configuration.
However, it is correct that in most cases, the section is selected via code. It can be changed via template edits though.
I'm a bit confused about what is going on here exactly though, as if you're just changing forums/ to discussions/, you would be better customizing the existing "forums" navigation tab and/or children, rather than creating an all new one.