HTTP header for identifying context at reverse proxy

Let's say I have reverse proxy which routes requests starting with /foo to http://somehost:8123

so http://proxy/foo/something ends up as http://somehost:8123/something.

In order for app somehost to provide proper URLs for the client it retrieves proxy:80 from X-Forwarded-Host or Host headers which are set by proxy.

But is there a similar standard way how to send the path prefix used in routing ?

Is this a proper way how to setup reverse proxy? I guess the other options is to introduce foo context path to app but I am not interested in this solution for now.