function submitPOST(elementId, api, callback) {

let input = document.getElementById(elementId).getElementsByTagName("input");

let body = {};
for (let i = 0; i < input.length; i++) {
    body[input[i].name] = input[i].value;
}

call('POST', api, body, callback);

xmlHttp.onload = function () {
    if (xmlHttp.status==200){
        callback(xmlHttp.response);
    }else{
        console.log(xmlHttp.response);
    }
};
xmlHttp.open(method, domainName+"/api/"+api, true); // true for asynchronous
xmlHttp.setRequestHeader("Content-Type", "application/json");
xmlHttp.send(JSON.stringify(body));