When you are running the Administration Window on the same server as the Inventu Viewer+ service, the default of localhost:83 should work OK, unless the Admin Http Port setting has been changed (that would the :83 portion of the address which would change).
If you are running the Administration Window on a different PC, enter the DNS name or IP Address of the machine running the Inventu Viewer+ service along with the configured port.
Example:
MyAppServer:83
