How to add custom links to Wordpress navigation bar (navbar)

This one is relatively easy. Go your wp-admin panel > Appearance > Editor

A list of files relating to your current themes (template) is on the right.

Click on Head (header.php)

In the editor box, find this line <?php wp_list_pages('title_li=&depth=-1'); ?>

Depends on your theme, that line might be different or the argument inside ( ) is different, but the key word you can look for is wp_list_pages(...) or wp_list_

For example, I want to add a Forum link to my navbar:

Below that line, I add <li><a href="yourlinkhere">Forum</a></li>

If you want to play around with it, you can put your external link before the Home page or in between your created pages.