Custom email template drupal8

Hi all,
How to alter email templates on user creation in drupal8,i am doing this by using hook_form_alter for alteration email subject and body:
function module_name_mail_alter(&$message) {
if ($message['key'] =='register_admin_created') {
$account = user_load_by_mail($message['to']);
$link = user_pass_reset_url($account, $options = []);
$message['body'] = $link;
$message['subject'] = t("Congratulations your account has been created");
}
I am following mime-mail readme.text,and my template file name is mimemail-messages--register_admin_created.html.twig ,But it my custom template is not reflected on user creation. Can any one help or suggest regarding this issue?????