(Drupal 8) Create a table in a database other than the default during module installation. Database must already be specified in settings.php.

d8module.install

<?php

/**
 * @file
 * Install, update and uninstall functions for the d8module module.
 */

function d8module_schema_otherdb() {
  $schema['mytable'] = array(
    'description' => 'My table description',
    'fields' => array(
      'myfield' => array(
        'description' => 'My field description',
        'type' => 'serial',
        'size' => 'medium',
        'not null' => TRUE,
        'unsigned' => TRUE,
      ),
    ),
    'primary key' => array('myfield'),
  );
  return $schema;
}

/**
 * Implements hook_install().
 */
function d8module_install() {
  \Drupal\Core\Database\Database::setActiveConnection('otherdb');
  $connection = \Drupal\Core\Database\Database::getConnection();

  $schema = d8module_schema_otherdb();
  foreach ($schema as $name => $table) {
    $connection->schema()->createTable($name, $table);
  }

  \Drupal\Core\Database\Database::setActiveConnection();
}

/**
 * Implements hook_uninstall().
 */
function d8module_uninstall() {
  \Drupal\Core\Database\Database::setActiveConnection('otherdb');
  $connection = \Drupal\Core\Database\Database::getConnection();

  $schema = d8module_schema_otherdb();
  foreach ($schema as $name => $table) {
    $connection->schema()->dropTable($name);
  }

  \Drupal\Core\Database\Database::setActiveConnection();
}