JSFiddle - React, Tailwind, and code Playground

HTML

<button class="finance-modal-btn">Apply Now</button>

<!-- Finance Modal -->
<div class="finance-modal">
  <div class="finance-modal-content">
    <span class="finance-modal-close">&times;</span>
    <iframe name="finance" id="finance" src="https://www.elbtools.com/secure/apply.php?elbt=1459262273170#" frameborder="0" border="0" cellspacing="0" style="width:100%; height: 600px;"></iframe>
  </div>
</div>

CSS

/* FINANCE MODAL */

.finance-modal {
  display: none;
  position: fixed;
  z-index: 100;
  padding-top: 100px;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgb(0, 0, 0);
  background-color: rgba(0, 0, 0, 0.4);
}

.finance-modal-content {
  background-color: #fefefe;
  margin: auto;
  padding: 20px;
  border: 1px solid #888;
  width: 80%;
  -webkit-animation-name: animatetop;
  -webkit-animation-duration: 0.4s;
  animation-name: animatetop;
  animation-duration: 0.4s;
}

/* Add Animation */

@-webkit-keyframes animatetop {
  from {
    top: -300px;
    opacity: 0;
  }
  to {
    top: 0;
    opacity: 1;
  }
}

@keyframes animatetop {
  from {
    top: -300px;
    opacity: 0;
  }
  to {
    top: 0;
    opacity: 1;
  }
}

/* The Close Button */

.finance-modal-close {
  color: #aaaaaa;
  float: right;
  font-size: 28px;
  font-weight: bold;
}

.finance-modal-close:hover,
.finance-modal-close:focus {
  color: #000;
  text-decoration: none;
  cursor: pointer;
}
.no-scrolling {
  overflow: hidden;
}

JavaScript

var modal = document.getElementsByClassName("finance-modal")[0];
var btn = document.getElementsByClassName("finance-modal-btn")[0];
var span = document.getElementsByClassName("finance-modal-close")[0];
var body = document.querySelector("body");

btn.onclick = function() {
    modal.style.display = "block";
    body.classList.add("no-scrolling");
}
span.onclick = function() {
    modal.style.display = "none";
    body.classList.remove("no-scrolling");
}
window.onclick = function(event) {
    if (event.target == modal) {
        modal.style.display = "none";
        body.classList.remove("no-scrolling");
    }
}