<button id="mybax">Open Message</button>
<div id="bax" class="netmod">
<div class="netmod-content">
    <span class="close">x</span>
    <p>Welcome!</p>
</div></div>

body {font-family: tahoma;}
.netmod {
  display: none; 
  position: fixed;
  z-index: 1; 
  padding-top: 100px; 
  left: 0;
  top: 0;
  width: 100%; 
  height: 100%; 
  overflow: auto; 
  background-color: transparent; 
}

.netmod-content {
  background-color: #960018;
  margin: auto;
  padding: 20px;
  border: 4px solid red;
  width: 400px;
height: 300px; 
 color: #fff;
}

.close {
  color: yellow;
  float: right;
  font-size: 28px;
  font-weight: bold;
}
.close:hover,
.close:focus {
  color: #fff;
  text-decoration: none;
  cursor: pointer;
}

var modal = document.getElementById("bax");
var btn = document.getElementById("mybax");
var span = document.getElementsByClassName("close")[0];
btn.onclick = function() {
  modal.style.display = "block";
}
span.onclick = function() {
  modal.style.display = "none";
}
window.onclick = function(event) {
  if (event.target == modal) {
    modal.style.display = "none";
  }
}

<iframe src="https://www.netbax.net/tab/javascript/modal.html" style="width: 550px; height:550px;" frameborder="0"></iframe>