How can I pass an arbitrary header field to an upstream system?

Use pair producers to copy the wanted header fields of the incoming request ($request/headers) to the request action configuration. Here we pass User-Agent and Accept:

<flow>
  <request>
  {
    "url": "http://httpbin.org/get",
    "headers": {
        {{: $request/headers/user-agent }}
        {{: $request/headers/accept }}
    }
  }
  </request>
</flow>

We can reduce this further to

    "headers": {
        {{: $request/headers/user-agent | $request/headers/accept }}
    }

or maybe even more readable using with:

    "headers": {
        {{with $request/headers }} {{: user-agent | accept }} {{end}}
    }