code: func postForm(ctx context.Context, client httpClient, endpoint string, values url.Values, intf interface{}, d debug) error {
  	reqBody := strings.NewReader(values.Encode())
  	req, err := http.NewRequest("POST", endpoint, reqBody)
  	if err != nil {
  		return err
  	}
  	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
  	return doPost(ctx, client, req, intf, d)
  }
docs: // post a url encoded form.