Please find this example code, You could use hidden form with POST to send data to that your URL like below:
function open_win()
{
var ChatWindow_Height = 650;
var ChatWindow_Width = 570;
window.open("Live Chat", "chat", "height=" + ChatWindow_Height + ", width = " + ChatWindow_Width);
//Hidden Form
var form = document.createElement("form");
form.setAttribute("method", "post");
form.setAttribute("action", "http://localhost:8080/login");
form.setAttribute("target", "chat");
//Hidden Field
var hiddenField1 = document.createElement("input");
var hiddenField2 = document.createElement("input");
//Login ID
hiddenField1.setAttribute("type", "hidden");
hiddenField1.setAttribute("id", "login");
hiddenField1.setAttribute("name", "login");
hiddenField1.setAttribute("value", "PreethiJain005");
//Password
hiddenField2.setAttribute("type", "hidden");
hiddenField2.setAttribute("id", "pass");
hiddenField2.setAttribute("name", "pass");
hiddenField2.setAttribute("value", "Pass@word$");
form.appendChild(hiddenField1);
form.appendChild(hiddenField2);
document.body.appendChild(form);
form.submit();
}