url_validate.js

// Validate using a custom URL (external server script)
function validates_instance_url( url, attr ) {
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
        if ( xhr.readyState === 4 ) {
            if ( xhr.status !== 200 || xhr.responseText === "false" || xhr.responseText === "0" ) {
                return false;
            } else {
                return true;
            }
        }
    }
    xhr.open( "GET", url + "?value=" + encodeURIComponent(attr), true );
    xhr.setRequestHeader( "Instance-Validation", attr );
    xhr.send();
}