You can use set your loglevel (REST Consume or REST Publish) to trace and see the actual JSON POST request.
If you add double braces, as Allard suggested, you can get the format you want:
Rest call action:
{{
"key" : {1}
}
result via trace log:
{
"key" : yourkeyhere
}
If I put all REST nodes onto trace then I see the JSON payload as the following:
Request content for POST request to https://url/ping.json HTTP/1.1
"key":"sfdsdfsdfsdfds"
It looks like it drops the braces? If I add the braces back in then I get a modeller error "Brace should be followed by a number of digits or another brace"
This error makes me think that I'm using the Custom request teamplte on the CallRest action incorrectly.