Websocket : Bad Handshake | Response 400 Bad Request

Got JSESSIONID after calling /api/session, then calling /api/socket but getting 400 Bad Request error. What's going wrong?

Cookie format that's getting passed in header : JSESSIONID=node0losdgdd5ed0je74vbnfqynav4559.node0; Path=/

Any help would be greatly appreciated! Here's the code chunk (golang)

    var addr = flag.String("addr", "demo3.traccar.org:443", "http service address")
    apiUrl := "https://" + *addr
    resource := "/api/session/"
    data := url.Values{}
    data.Set("email", "--email--")
    data.Set("password", "--pass--")

    u, _ := url.ParseRequestURI(apiUrl)
    u.Path = resource
    urlStr := u.String()

    client := &http.Client{}
    r, _ := http.NewRequest(http.MethodPost, urlStr, strings.NewReader(data.Encode()))
    resp, _ := client.Do(r)

    header := http.Header{}
    header.Add("Cookie", resp.Cookies()[0].String())
    fmt.Println(resp.Cookies()[0])

    interrupt := make(chan os.Signal, 1)
    signal.Notify(interrupt, os.Interrupt)

    uNew := url.URL{Scheme: "ws", Host: *addr, Path: "/api/socket"}
    log.Printf("connecting to %s", uNew.String())

    d := websocket.Dialer{}
    c, resp, err := d.Dial(uNew.String(), header)
    if err != nil {
        log.Fatal("dial:", err, resp)
    }
    defer c.Close()

Without the :443 port, getting 503 Service Unavailable

What's the response payload?

Thanks Anton, got the fix : Added the cookies to websocket dialer and getting the correct response now
dialer.Jar.SetCookies(&uNew, []*http.Cookie{&cookie})