How would I change the src of an iframe with a text input?

So, Mojang released a version of Minecraft for the browser and I've been trying to add it to my website. I successfully did that with an iframe but people wanted multiplayer. Since multiplayer works through links, the goal would be to input the link they receive from someone else and change the iframe from the standard one to the new multiplayer one.

What I've tried is to get the user input, assign it to a variable, and use document.write to rewrite the existing iframe with the new src.

Here's what I've got. Note: it's in a modal.

input[type=text], input[type=password] {
  width: 300px;
}
.cancelbtn {
  width: auto;
  padding: 10px 18px;
  background-color: #f44336;
}

/* Center the image and position the close button */
.imgcontainer {
  text-align: center;
  margin: 24px 0 12px 0;
  position: relative;
}

img.avatar {
  width: 40%;
  border-radius: 50%;
}

.container {
  padding: 16px;
}

span.psw {
  float: right;
  padding-top: 16px;
}

/* The Modal (background) */
.modal {
  display: none; /* Hidden by default */
  position: fixed; /* Stay in place */
  z-index: 1; /* Sit on top */
  left: 0;
  top: 0;
  width: 100%; /* Full width */
  height: 100%; /* Full height */
  overflow: auto; /* Enable scroll if needed */
  background-color: rgb(0,0,0); /* Fallback color */
  background-color: rgba(0,0,0,0.4); /* Black w/ opacity */
  padding-top: 60px;
}

/* Modal Content/Box */
.modal-content {
  background-color: #fefefe;
  margin: 5% auto 15% auto; /* 5% from the top, 15% from the bottom and centered */
  border: 1px solid #888;
  width: 80%; /* Could be more or less, depending on screen size */
}

/* The Close Button (x) */
.close {
  position: absolute;
  right: 25px;
  top: 0;
  color: #000;
  font-size: 35px;
  font-weight: bold;
}

.close:hover,
.close:focus {
  color: red;
  cursor: pointer;
}

/* Add Zoom Animation */
.animate {
  -webkit-animation: animatezoom 0.6s;
  animation: animatezoom 0.6s
}

@-webkit-keyframes animatezoom {
  from {-webkit-transform: scale(0)} 
  to {-webkit-transform: scale(1)}
}
  
@keyframes animatezoom {
  from {transform: scale(0)} 
  to {transform: scale(1)}
}

/* Change styles for span and cancel button on extra small screens */
@media screen and (max-width: 300px) {
  span.psw {
     display: block;
     float: none;
  }
  .cancelbtn {
     width: 100%;
  }
}
.button {
  color: white;
  background-color: #4CAF50;
  border-color: #ccc;
  font-size: 14px;
  font-weight: 400;
  border: 1px solid transparent;
  border-radius: 4px;
  cursor: pointer;
  line-height: 1.42857143;
  text-align: center;
  white-space: nowrap;
  vertical-align: middle;
  padding: 6px 12px;
}

<div id="container">
<div align="center"><iframe id="change" name="change" width="800" height="500" src="//classic.minecraft.net"></iframe></div>
 <div>
 <button class="button" id="button"><b>Fullscreen</b> </button>
 <button class="button" id="modal" onclick="document.getElementById('id01').style.display='block'"><b>Multiplayer</b></button>
 </div>
 <script src="/fullscreen.js"></script>
 <script>
   function changeIframe() {
  document.write(" <iframe  id='change' name='change' src='#" + input + "' width='800'  height='500'   allowfullscreen></iframe>");
}
 </script>
 <script>   
 var input = document.getElementById("detroit");
 </script>
 <div id="id01" class="sprckn-modal">
  
<form class="modal-content animate">
    <span onclick="document.getElementById('id01').style.display='none'" class="close" title="Close Modal">&times;</span>
  
  
  <div class="sprckn-modal-content">
    <label for="link"><b>Paste Multiplayer Link Here</b></label>
    <center><input type="text" onsubmit="changeIframe()" id="detroit" placeholder="https://classic.minecraft.net/?join=somelettersand#s" name="link" required></center>
  </div>
  
  <div class="container" style="background-color:#f1f1f1">
    <center><button type="button" onclick="document.getElementById('id01').style.display='none'" class="cancelbtn">Cancel</button></center>
</form>
  </div>
</div>
`

Absolutely no part of the iframe changes, but my website url does, with a ?link=(linkhere). It can be better shown off at https://sprocken.com/game/minecraft/.

Solution

Change the existing iframe source:

function changeIframe(url) {
  document.getElementById('change').src = url;
}

Use the onchange event instead of onsubmit:

<input type="text" id="minecraftInput" onchange="changeIframe(this.value)" id="detroit" placeholder="https://classic.minecraft.net/?join=somelettersand#s" name="link" required>