Thank you very much to all people you helped me here. I've spent the last two days trying to understand what you had advised me to do with my meager knowledge and I finally got a satsifactory solution :

<div align = "left">
  <form>
      <div class = "group">      
      <input type = "password" name = "password" id = "accesspasswd" required>
      <span class = "highlight"></span>
      <span class = "bar"></span><IMG SRC = "images/padlock.png" onclick = "onSubmit()">
    </div>
  </form>
</div>
</punlock>
</div>
<script>
     function onSubmit() {
const input = document.getElementById('accesspasswd').value.trim();
window.location.href = `welcome${input}.html`;
     };
</script>

If the page welcomeMYPASSWORDS.html exists, this code redirects me to it. And if it doesn't exist, my Error404 redirects me to my index.html ! In any case, the passwords are not longer visible in the source code of the pages. So, that's PERFECT !

Thank you so much ! ^^