So presumably you want to add an event listener to the button so that it submits when clicked. You can go about that by adding an event listener to the element between two script tags <script></script> or in your .js file. The code would look like:

document.getElementById("send-payment").addEventListener("click", function() {
     /*
          Action Here...
     */
     alert("Request Sent!") // This will throw up an alert to the user.
})

From what I gather this should be a simple solution, you can find more documentation here: https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener

You need a space to display the message "Request sent". Here I use the p tag

<div class="box-element hidden" id="request-info">
  <button id="send-payment">Send request</button>
  <p id="response"></p>
<script>
 document.getElementById("send-payment").addEventListener("click", function(){
 document.getElementById("response").innerHTML = "Request sent";
})
</script>

Here is the link on w3schools: https://www.w3schools.com/jsref/met_document_addeventlistener.asp

Make this

const button = document.getElementById("send-payment");

button.addEventListener("click", function() {
  document.getElementById("request-info").style.display = "block";
})

#request-info {
  display: none;
  color: green;
  font-size: 12px;
  margin-bottom: 10px;
}

<div id="request-info">Request sent</div>
<button id="send-payment">Send request</button>