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}}
}