generateHmac static method
Sign the payload with your appSecret. Otherwise the server will deny the request
Implementation
static Future<String> generateHmac(String data, String appSecret) async {
var key = utf8.encode(appSecret);
var bytes = utf8.encode(data);
var hmacSha256 = Hmac(sha256, key);
var digest = hmacSha256.convert(bytes);
return digest.toString();
}