This is possible now. Here is an example https://github.com/wg/wrk/blob/master/scripts/post.lua.
wrk.method = "POST"
wrk.body = "foo=bar&baz=quux"
wrk.headers["Content-Type"] = "application/x-www-form-urlencoded"
save this in a *.lua script and pass it into your command line test with the -s flag.