Code for workers to submit HIT on MTurk isn't working ("There was a problem submitting this HIT")

Question

I have a HIT that (I think) is internal, not external, in the sense that I'm copy/pasting my html and JS code in the "Design and Layout" tab of the "Create project" process on the requester sandbox website for MTurk (https://requestersandbox.mturk.com/).

Most of the code works but I can't figure out how to write the code so that workers can successfully submit the HIT.

I've tried this code I found online:

<div class="page" id="submission-page" style="display: none;">
    <div class="cell">
        <div class="warning">You've completed the HIT -- Thanks for your help!</div>

        <form id="submission-form" method="post" action="https://workersandbox.mturk.com/mturk/externalSubmit">
            <input type="hidden" id="assignmentId" name="assignmentId" value="">
            <input type="submit" value="Submit HIT">
        </form>
    </div>
</div>

I've also tried a generic submit type in <form> tags without any post method of action.

Both return "There was a problem submitting this HIT".

What is the simplest way to succesfully end the HIT (so that it is considered "submitted" by MTurk) when a worker presses a button on the last page of the HIT?