How to add keyboard shortcuts for switching to workspaces 5 and above

Greetings,

System: Debian Testing, AMD64.

After several years of using Fluxbox exclusively, I want to try Gnome
again but need some help with configuration.  

I've created 8 workspaces.  Using Settings->Keyboard->Shortcuts, I was
able to create shortcuts for workspaces 1-4 without any problem;
however, workspaces 5 and higher require custom shortcuts.  What is the
command to enter for switching to workspace 5, for example?  My
searches in the help documents and with Google have turned up empty.

Thanks in advance for any help.

Tom Ashley