Javascript timeout problem with Flask backend

Question

I am trying to write JS to wait and redirect after button click. Note this button is also posting message to backend Flask to trigger a function.

My HTML code:

<form action="{{ url_for('start_task1') }}" method="post">
<button type="submit" onclick="pageRedirect();">Submit</button>
</form>

<div id="message"></div>
    <script>
 function pageRedirect(){
   var delay = 6000; // time in milliseconds
   // Display message after button click
   document.getElementById("message").innerHTML = "Please wait, you are redirecting to the new page.";
   setTimeout(function(){
       window.location.href = "/flaskendpoint";
   },delay);
}
   </script>

Expected behavior when I was trying:

The way I thought it should have worked is that after button click, the page displays the "message" and waits for 6 seconds and then it redirects to another view function / endpoint like this --> (/flaskendpoint) in my Flask app.

Problem:

Problem I am getting is that after button click the "message" appears on the page only momentarily and then it vanishes, and the page does not redirect.

There are no errors in console.