Officialy the URL for login to yur Ghost Admin Panel is:
But actually, the possibilities are quite more hidden under the hood.
You can login to the admin panel using any of the following urls:
http://your-ghost.com/admin http://your-ghost.com/signin http://your-ghost.com/wp-admin http://your-ghost.com/dashboard http://your-ghost.com/ghost-admin
All will work and it is an official way. You should try it !