how we can add expiry to the file_url given by back4app?

Question

since it is not possible to access the storage bucket in which back4app stores the file, i am failing to add custom expiry to the url provided by back4app for that file because, the base url has no expiry provided.

expiration := expiry.Unix()
signedURL := fmt.Sprintf("%s?expires=%d", filePath, expiration)
mac := hmac.New(sha256.New, []byte(secretKey))
mac.Write([]byte(signedURL))
signature := base64.URLEncoding.EncodeToString(mac.Sum(nil))
url = fmt.Sprintf("%s&signature=%s", signedURL, signature)

i tried this code and other similar codes, but nothing worked. it will be awesome if it can be done using rest api