You can set your own custom Magento admin URL in:
System -> Configuration -> [ADVANCED] -> Admin -> Admin Base URL
- Use Custom Admin URL = (YES)
- Custom Admin URL = https://backend.domain.com
NOTE: Make sure you have added a ServerAlias directive to your virtual host configuration for backend.domain.com and that you have a SSL cert or wildcart setup for HTTPS traffic on that subdomain or you will lock yourself out the admin.
Rules for your .htaccess file to prevent duplicate urls and redirect directly to admin dashboard: