How to connect easypaisa/jazzcash payment gateway in flutter? [closed]

I have implemented the jazz cash payment gate through REST APi,Step 1: sign into sandbox jazz cash accountStep 2: select the REST API version 1.1Step 3: copy the merchant id, password, and the integrity salt id

here is the code below and the tutorial: https://www.youtube.com/watch?v=wW9V7IVjpic&t=10

 payment() async{var digest;String dateandtime = DateFormat("yyyyMMddHHmmss").format(DateTime.now());String dexpiredate = DateFormat("yyyyMMddHHmmss").format(DateTime.now().add(Duration(days: 1)));String tre = "T"+dateandtime;String pp_Amount="100000";String pp_BillReference="billRef";String pp_Description="Description";String pp_Language="EN";String pp_MerchantID="your id";String pp_Password="your password";String pp_ReturnURL="https://sandbox.jazzcash.com.pk/ApplicationAPI/API/Payment/DoTransaction";String pp_ver = "1.1";String pp_TxnCurrency= "PKR";String pp_TxnDateTime=dateandtime.toString();String pp_TxnExpiryDateTime=dexpiredate.toString();String pp_TxnRefNo=tre.toString();String pp_TxnType="MWALLET";String ppmpf_1="4456733833993";String IntegeritySalt = "your key";String and = '&';String superdata=    IntegeritySalt+and+        pp_Amount+and+        pp_BillReference +and+        pp_Description +and+        pp_Language +and+        pp_MerchantID +and+        pp_Password +and+        pp_ReturnURL +and+        pp_TxnCurrency+and+        pp_TxnDateTime +and+        pp_TxnExpiryDateTime +and+        pp_TxnRefNo+and+        pp_TxnType+and+        pp_ver+and+        ppmpf_1;var key = utf8.encode(IntegeritySalt);var bytes = utf8.encode(superdata);var hmacSha256 = new Hmac(sha256, key);Digest sha256Result = hmacSha256.convert(bytes);var url = 'https://sandbox.jazzcash.com.pk/ApplicationAPI/API/Payment/DoTransaction';var response = await http.post(url, body: {  "pp_Version": pp_ver,  "pp_TxnType": pp_TxnType,  "pp_Language": pp_Language,  "pp_MerchantID": pp_MerchantID,  "pp_Password": pp_Password,  "pp_TxnRefNo": tre,  "pp_Amount": pp_Amount,  "pp_TxnCurrency": pp_TxnCurrency,  "pp_TxnDateTime": dateandtime,  "pp_BillReference": pp_BillReference,  "pp_Description": pp_Description,  "pp_TxnExpiryDateTime":dexpiredate,  "pp_ReturnURL": pp_ReturnURL,  "pp_SecureHash": sha256Result.toString(),  "ppmpf_1":"4456733833993"});print("response=>");print(response.body);

}