How to link category to other site, how to link category to static page

1. Why make Forum a category, make it a page and see 2 below. If that doesn’t fit your need, then you’ll need to put the link in a Category Template.
2. likely answered by How to easily display links to both Pages and Categories in the blog navigation header?

3. Review template tag, wp_list_categories(), for orderby options.

Thanks Michael but it doesn’t help me much.
1. Forum as a page link is ok, but not enough, i would like to have it big as a part of category menu. I tried to put the link in code in archive.php ( there is no category-6.php or category.php provided in my template) but it doesn’t work. I tried to use this code:

<?php
  if( $iCategory == 22 ){ // id of category with link
   header( 'https://mypage.com/' ;); // external link
   exit;
  }
?>

Maybe this code is wrong or i am trying to put it in wrong place.

2. I need to link Category to specific page not page to category. Using this method I can link pages to categories, but how to do it in other direction?

thank you for help, regards.

any ideas please

The way that I put posts inside static pages is like this:
Creat your categories. Then create the pages. You want to install the plugin Page Links To.
go to your live site, and click on the category that you wish to be in the page and copy the URL.
Go to the page and insert the link at the bottom of the page that says Page Link To and then publish.
When you click on the page, that should bring you to the category page with all of the posts in that category.