Browser password manager not popping up on HTML form submit

Question

I have seen this question asked a couple of times, but none of the proposed solutions are working for me.

I have a simple html file containing the following form:

<form action="/" method="POST" onsubmit="return false;">
    <input type="text" name="username">
    <input type="password" name="password">
    <input type="submit" name="submit">
</form>

I expect the Browser password manager to pop up when I hit submit.

It feels like I am overlooking something simple, but I can not get any browser to pop up with the question to save the credentials in the password manager. I have tried this in Chrome, Edge, Firefox and Vivaldi.

I do get the pop up on other websites.

Even asked ChatGPT, who came with the following solution:

<form>
    <input type="text" name="username" id="username" placeholder="Username">
    <input type="password" name="password" id="password" placeholder="Password" autocomplete="on">
    <input type="submit" value="Log In">
</form>

But this also did not work.

Any advice would be greatly appreciated!