theming – How I can send html mail using htmlmail module in drupal 8

I am trying to send html mail using drupal service plugin.manager.mail but I am not able to send html mail.

Controller file:

    $module='module_name';
    $key='usernamerecovery';
    $to = $mail;
    $langcode ='en';
    $params('user_name') = $username;
Drupal::service('plugin.manager.mail')->mail($module, $key, $to, $langcode, $params, $reply = NULL, $send = TRUE);   

.module file

function module_name_mail($key, &$message, $params) {
$site_name = Drupal::config('system.site')->get('name');
$site_mail = Drupal::config('system.site')->get('mail');

switch ($key) {
case 'usernamerecovery':
  $message('headers')('Reply-To') = $site_mail;
  $message('headers')('From') = $site_name . '< ' . $site_mail . ' >';
  $message('subject') = t('@site', ('@site' => Drupal::config('system.site')->get('name').' Username Request'));
  $message('body')() = t('Hi '.$params('user_name').',');     
  $message('body')() = t('Username: '.$params('user_name'));
  $message('body')() = t('If you have any questions, do let us know.');
  $message('body')() = t('Best,');
  $message('body')() = t('@site', ('@site' => $site_name.' Support Team'));
  break;
}
}

Template file : htmlmail–module_name.html.twig

which I Placed in module/custom/module_name/template/htmlmail–module_name.html.twig –

in this file I copied and past code of htmlmail module template file htmlmail.html.twig