Serialise JSON in REST call using the "Call Rest" action

Is there a way to see the JSON being submitted while using the Rest action within Mendix 7

I am trying to submit the following JSON string via post

{"key": "sfdsdfsdfsdfds"
}

Within the action I interpret it as  where index 1 is the $Key string value.

"key":"{1}"

The response i get, however is: 

{"status":"error","code":-1,"name":"ValidationError","message":"You must specify a key value"}

Debug shows the $Key variable is populated correctly. 

I would like to see the actual JSON being submitted so that I can see what the exact problem is.