but this seems like you're pushing NS data to some downstream system (or will be in prod)
the part you need to test isn't really the http post, I wouldn't think?
you need to make sure the data you're getting out is valid and formatted correctly for the receiving system... and you can only really do that from the receiving system side.
but to the extent you can do it in NS... just massage your data as needed and log it, you don't need to actually post it to anywhere if the target system doesn't have a non-prod environment to test against, logging the JSON is as good as it gets.