let call_backend = function(e) {
    type = event.target.getAttribute('name');

    var url_base = document.forms["link"]["url-field"].value;
    var run = document.forms["link"]["run-field"].value;
    var link = document.forms["link"]["link-field"].value;

    x = new XMLHttpRequest();
    x.onreadystatechange = function() {
        if (x.readyState === 4) {
            console.log("response: " + x.responseText);
            document.getElementById("result").innerHTML = x.responseText;
        };
    };

    var url = "http://" + url_base + "/link/" + (type === "start" ? "start" : "handoff") + "/" + run + "/" + link;
    // console.log(url);
    x.open("GET", url, true);
    x.send();
};