So, I am trying to post a subject and the content of the textarea whose id is "main" and based on the subject, the php will insert the content to the right database. But something goes wrong and the insertion does not happen, probably because I am doing something wrong with the url. Here is my code:

function saveContent(){
    const note = document.getElementById("main").value;
    console.log(note);
    const subjectToSend = subject;
    let param = "content="+note+"-subject="+subjectToSend;
    const xmlhttp = new XMLHttpRequest();
    xmlhttp.onload = function() {
    }
    xmlhttp.open('POST', 'index.php');
    xmlhttp.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
    xmlhttp.send(param);
}