PayuMoney integration not working in flutter app

Question

I am trying to use PayUMoney in my flutter application, I tried using two packages:

1. https://pub.dev/packages/payumoney/example
2. https://pub.dev/packages/payu_money_flutter/example

Following is the code for package 1:

Map map = {
      "txnid":
          "13",
      "amount": "2345",
      "name": "Sample",
      "email": "sampleemail",
      "phone":
          "sample_phone",
      "productName": "sample_name",
      "hash":
          "sample_hash"
    };
    String response = await Payumoney.paymentStart(map);

Whenever I run I get the error:

1. Unhandled Exception: MissingPluginException(No implementation found for method getPayu on channel payumoney)

Following is the code for package 2:

var myResponse = await payuMoneyFlutter.startPayment(
        txnid: "id",
        amount: "amount",
        name: "Sample",
        email: "sample_email",
        phone: "sample_phone",
        productName: "sample_name",
        hash: "sample_hash";

The error I get for the above code is:

MissingPluginException(No implementation found for method startPayment on channel payu_money_flutter)

How do I integrate PayUMoney in my application?