what is the id for server rewards like RP 

No id needed, simply set the "Use reward points" setting under General Settings in the config to "true" and it will ignore whatever you have set for "Currency item ID" and use RP instead. Your config should look like this:

{
  "General settings": {
    "Currency item ID (if not using economics)": -932201673,
    "Use economics balance (requires economics plugin)": false,
    "Use reward points (requires server rewards plugin)": true
  },