How to configure Trusted_Host_Settings for subdomains?
It's actually documented in settings.php file as in examples:

Drupal 8.5.x
https://cgit.drupalcode.org/drupal/tree/sites/default/default.settings.php?h=8.5.x#n712

&lt;?php
$settings['trusted_host_patterns'] = array(
  '^example\.com$',           // Matches naked domain only.
  '^.+\.example\.com$',       // Matches any subdomains.
  '^subdomain\.example\.com', // Matches a specific subdomain.
);


You can easily define any naked or sub-domain, as long as defined regex pattern matches them correctly.