I have REST Service which works when I use the URL with the Userid manually added to the end i.e.

In the case above i add 'US123ABC' to the URL and it works, but of course this will never be the same so I need to pass this dynamically.
In ESB I have a message type with my needed field:

But when I try and include this in my REST receiver channel it doesn't work:

The above is my attempt to try and pass Userid dynamically to the URL but it doesn't work.
All help is greatly appreciated.