form := url.Values {
    "username": {"Jack"},
}

proxyUrl, err := url.Parse(mass[bproxy]) //массив с прокси серверами
httpClient := &http.Client { Transport: &http.Transport { Proxy: http.ProxyURL(proxyUrl) } }

blose := bytes.NewBufferString(form.Encode())
httpr, err := httpClient.Post(host, "application/x-www-form-urlencoded", bclose)
httpr.Close = true;

if err != nil {
    panic(err)
}