You can create .htaccess rewrite rule for this.
You can make the url look like this
http://[domain]/[secret_admin]
Rewrite rule will look like this:
RewriteEngine On
RewriteRule ^secret_admin$ /index.php?rt=index/login&s=HY_admin [R=301,QSA,L]
NOTE: This is an example rule. You need to adjust it to your specific settings.