That is a JSON object. They're Key value pairs. So:
{
"peripheral_id": "1",
"sender_id": "${loginId}"
}
I'm not sure this can really be done without a gadget. I'm not sure what it wants you to do here, but you can't post body in a Finesse Workflow. I'm not sure if it intends for you to use the mediasense gadget (I haven't messed with the Mediasense gadget or even read up on it) or what.
There are a couple of ways I could see this working:
First, you write a gadget that handles the workflow action ("other") and then uses gadgets.io.makeRequest to perform the REST action specified in the guide (of course, sending the JSON object above).
Second, you write a gadget that sees / monitors dialogs and instead of triggering recording with a workflow, you start and stop the recording using gadgets.io.makeRequest on the handler functions for DialogAdd and DialogDelete that you would presumably add to the Dialogs collection and New dialog respectively.
In the end, Cisco needs to update / correct the documentation. The Java/JSON for the metadata version of the post body below this one isn't even formatted correctly as far as I can tell.