Send and receive json messages with nighthawk

Hi, Is there a way to POST and GET requests with json as a body ?
Something like running nigthawk client with:
--request-method POST --request-header 'Content-Type: application/json' and --request-body '{foo: bar}'
I see in Readme there is only --request-body-size that is int.
Same thing for nighthawk server is there a way to get json or other message instead of multiple a's ?
Thanks :)