editor.php

<form action="/action_page.php" id="usrform">
    Name: <input type="text" name="usrname">
    <input type="submit">
</form>
<br>
<textarea rows="4" cols="50" name="comment" form="usrform">
Enter text here...</textarea>

<script src="https://unpkg.com/stackedit-js@1.0.7/docs/lib/stackedit.min.js" type="module"></script>
<script>
    // Import the lib.
    import Stackedit from 'stackedit-js';

    // Get the textarea.
    const el = document.querySelector('textarea');

    // Create the Stackedit object.
    const stackedit = new Stackedit();

    // Open the iframe
    stackedit.openFile({
        name: 'Filename', // with a filename
        content: {
            text: el.value // and the Markdown content.
        }
    });

    // Listen to StackEdit events and apply the changes to the textarea.
    stackedit.on('fileChange', (file) => {
        el.value = file.content.text;
    });
</script>