How to make a hyperlink conditional according to whether you are logged in or not

You need to be able to edit the theme itself (or as a child theme) to make that change – obviously the theme options aren’t giving you that option.

Chances are that in your header.php or similar you are going to find code that pulls those values from the Theme Options page. That’s what you need to edit. When you find that code, you’ll want to do a conditional

 if( is_user_logged_in() ){
   echo '/product-category/aktiviteter/';
 }
 else{
   echo '/login/';
 }

wherever that URL is being set in the template.