JSはjson形式のPOSTリクエストメソッドを使用します

<script type="text/javascript">

var us = new US();
var xhr = new XMLHttpRequest();
xhr.open("POST", "/searchguard/api/v1/auth/login", true);
xhr.setRequestHeader("Content-type", "application/json");
xhr.setRequestHeader("kbn-version", "5.3.0");
xhr.onreadystatechange = function() {
if (xhr.readyState == 4) {
if (xhr.status == 200) {
window.location.href = us.nextUrl;
}
}
};
xhr.send(JSON.stringify({
"username" : us.u,
"password" : us.p
}));
</script>