makingrequest.js

const body = {
    message: 'hi'
}

const data = new FormData();
data.append( "json", JSON.stringify( body ) );

fetch("/echo/json/", {
    method: "POST",
    body: data
})
.then(function(res){ return res.json(); })
.then(function(data){ console.log(JSON.stringify(data)) })