lwp-request in shell: how to make POST request with body?

I use simple shell script to test some http server that process POST requests. Usually it looks like:

echo "param1=value1&param2=value2" | POST localhost/service

But now I want to pass also some json in POST body and that's where I missing point completely.

man POST and google did't help much also.

Seems it must be either very simple or completely impossible.

Thanks for help.