How do I define an external URL in settings.php and call it in a custom module?
Trying using the full path of the class:

\Drupal\Core\Site\Settings::get('external_url')


In Drupal 8, an ideal place for custom variables is in config files. Create a yml file in the config directory and import the configuration.

my_variables.settings.yml

external_url: 'url-string-here'


Then to fetch the values, it's: 

\Drupal::config('my_variables.settings')-&gt;get('external_url')


https://www.drupal.org/docs/8/converting-drupal-7-modules-to-drupal-8/step-4-convert-drupal-7-variables-to-drupal-8