How to get the return of a javascript variable with Flutter Web

Question

Colleagues, please, I need to receive the return variable from this encryption library below. My application is in Flutter Web, and I don’t know how to include Javascript in Flutter Web. I’ve done a lot of research, but I really haven’t got any results

The Library of Cryptography:

<script type="text/javascript" src="https://sandbox.boletobancario.com/boletofacil/wro/direct-checkout.min.js"></script>

Getting the hash:

In this step you will need to inform the card data to our library, which will validate card data, encrypt it and return a hash.

Below is an example of our library’s use of how to get the credit card hash:

 <script type="text/javascript">
  var checkout = new DirectCheckout('PUBLIC_TOKEN', false); 
  function generateHash() {
    var cardData = {
        cardNumber: '1111 2222 3333 4444',
        holderName: 'Nome do Titular',
        securityCode: '123',
        expirationMonth: '12',
        expirationYear: '2025'
      };
  }
  checkout.getCardHash(cardData, function(cardHash) {
      /* Sucesso - A variável cardHash conterá o hash do cartão */
  }, function(error) {
      /* Erro - A variável error conterá o erro ocorrido ao obter o hash */
  });
</script>