How to add parameters in request.content

[plvndinesh]

I have created a reverse proxy and body parameters are

{"name":"xyz","Id":"123"}

but by modifying the payload in javascript policy

var payload = JSON.parse(context.getVariable("request.content"));
payload.key ="xyzkey";
context.setVariable("request.content",JSON.stringify(payload));

or by using assign message policy

<Set>
  <Payload contentType="application/json">{payload}<Payload/>
<Set>

by modifying the request.content

Request sent to target server

url become blank.

How to overcome the problem?