Configuration of multi-site on Drupal 8 is similar to previous Drupal versions. However, unlike previous versions, Drupal 8 requires multi-site to be explicitly enabled before additional sites are recognized.
Enabling multi-site with sites.php
Multi-site requires the existence of the sites.php file in the sites directory. This file does not exist by default. If this file does not yet exist, copy sites/example.sites.php as follows:
$ cp sites/example.sites.php sites/sites.php
You should edit the sites.php file for your cases, for example:
$sites = array(
'site1.yourdomain.com' => 'site1',
'site2.anotherdomain.net' => 'site2'
Each site in a multi-site installation is signified by a directory under the sites directory. Drupal is installed with a sites/default directory. Additional sites are represented by creating additional directories. For example:
Each new site directory must have a settings.php file in order for Drupal to recognize it. On initial installation of a site, copy sites/default/default.settings.php as follows:
$ cp sites/default/default.settings.php sites/[new site directory]/settings.php
$ cp sites/default/default.services.yml sites/[new site directory]/services.yml
Then, you should visit the url domain and start to install a new Drupal site, for example:
Also, you should make sure your web server (nginx, apache or IIS) already finished the setting and your DNS records should be mapped ready.