Edit in JSFiddle

var url = "https://jsonplaceholder.typicode.com/posts"

var http = new XMLHttpRequest()

http.onreadystatechange = function() {
    if (this.readyState != 4) {
        return
    }
    
    document.getElementById("received").innerHTML = this.responseText
}
    
function laheta() {
    var data = {
        title: document.getElementById("text").value,
        body: "hello world"
    }
    
    http.open("POST", url)
    http.send(JSON.stringify(data))
}
<input type="text" id="text"/>
<button onclick="laheta()">Lähetä!</button>
<p id="received">
</p>