To submit a form to a new window in post method,

HTML

<form name="form_message_center_popup" id="form_message_center_popup" method="post" action="/folder/file.ext">
    <input type="text" name="user_name" id="user_name" value="abc"/>
    <input type="text" name="clientid" id="clientid" value="efg"/>
    <input type="button" onclick="form_submiter()" value="submit"/>
</form>

JavaScript

function customer_support_msg() {
    var customer_support = window.open("about:blank", "_form", "location=no, menubar=no, toolbar=no, fullscreen=yes, scrollbars=yes, resizable=yes, width=800, height=600, replace=true");
    document.forms["form_message_center_popup"].target='_form';
    document.forms["form_message_center_popup"].submit();
    return false;
}