Forward plumbeR request to other endpoint in mounted router

This is perhaps a stupid question, but I can't seem to figure it out.

The following piece of code creates a router that exposes additional endpoints (such as /process) from inside the API where the function is run:

#* @plumber
function(pr) {
  modulateapi <- plumber::plumb("/path/to/other/plumber.R")
  pr %>%
    pr_mount("/modulate/", modulateapi )
}

Now, what I would like to do is to call the endpoints in the mounted router from inside the API where the mount exists. So for example if the above code exists in a file called 'all.R', then that file should also contain a function/endpoint that forwards the request to one of the mounted endpoints such as /process (i.e. the complete path would be /modulate/process):

# Redirect to modulate
#* @get /modforward
function(req, res){
    # Forward the request here to /modulate/process
}

So the question is: what should the code look like in the body of '/modforward' so it simply forwards the request to /modulate/process ?

I tried setting status = 303 and update 'Location' in the header of the response (res) to point to /modforward which correctly forwards, but that does not contain the request. I have hit rock bottom in trying to figure it out, so hopefully someone knows if this is even possible.