The tootip display: 

Url prefix forced onto workspaces. Allow you to specify the beginning of the URL of any workspace.<br/>An empty value will allow any value from the front.<br/>A prefix value of 'http://localhost/site/' will force workspaces to have an URL like 'http://localhost/site/xxx'

If you follow this tooltip you got this error:

The url is invalid. It must started with http:// or https://

But you can set 'http://localhost/site' in this config

Good luck for our user to configure it ! And the same in french !