Integrating Dialogflow WebDemo in website?

I have integrated the bot in the website but the issue is, when the user gives the answer and press the enter then I should call the API. So, how to find that user presses the enter?

<div id="myModal1" class="modal fade" id="exampleModalCenter" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true">
    <div class="modal-dialog modal-dialog-centered" role="document">
      <div class="modal-content chat-box">
        <div class="modal-header">
          <h5 class="modal-title" id="exampleModalLongTitle">Bot</h5>
          <button type="button" class="close" data-dismiss="modal" aria-label="Close" onclick="closeModal1()">
              <span aria-hidden="true">&times;</span>
            </button>
        </div>
        <div class="modal-body">

            <iframe
            allow="microphone;"
            width="350"
            height="430"
            src="">
        </iframe>
        </iframe>
        </div>

      </div>
    </div>
  </div>