How prevent reload after post request?

I am trying to make a post request to the server and do something with the response. Things seem to work on the server-side. The problem is that the page reloads upon completion of the response and I cannot do anything with the response.

The common suggestions are using button or preventDefault. These suggestions do not solve the problem: as you can see below, the input type is button (not submit) and preventDefault() on the event does not work.

Does anyone have an idea about what I am missing?

 <form id="modelCodeForm">
    <label for="codehere"></label>
    <div id="modelAreaDiv">
      <textarea id="modelArea" cols="60" rows="20">
stuff
      </textarea>
      <br>
      <input id="submitUserModel" type="button" value="run on server">
  </div>
  </form>

function initializeUserModel(){
  let model = document.getElementById("modelArea").value;
  fetch('http://localhost:8080/', {
      method: 'post',
      headers: {'Content-Type': 'text/plain'},
      body: model
    })
      .then(response => response.json())
      .then(data => {
        console.log(data);
      }).then(console.log("received!"))     
}