How to add Body content to POST Rest Service

In a POST REST service i need to add in the  Body Content Request: 

"command=returnBalances&nonce=23"             (no more, no less, no json, just this text)

with nonce being a unix timestamp.

I got the service to work by manually changing the nonce. But I want to set nonce as a parameter.
I cant seem to figure out how to add this string to the body content.


On a Rest service call, you have the OnBeforeRequest action that you can use to customize your request (CostumizedRequest.RequestText).