How use ajax post in backend (post edit)

Hi, I successfully make a ajax post request from frontend:

<script type="text/javascript">
    document.addEventListener('DOMContentLoaded', function() {
        var ajaxUrl = "<?php echo admin_url('admin-ajax.php'); ?>";

        document.getElementById('myButton').addEventListener('click', function(e) {
            e.preventDefault();

            var message = document.getElementById('ai-input-title').value;

            var data = new FormData();
            data.append('action', 'send_request_to_backend'); 
            data.append('message', message); 

            fetch(ajaxUrl, {
                method: 'POST',
                body: data
            })
            .then(response => response.text())
            .then(response => {
                alert('answer: ' + response);
            })
            .catch(error => console.error('err:', error));
        });
    });
</script>

But I need run this code in WP backend (while post add / edit). When I run this code in post add / edit screen, I have 404 error…. In error message I see:

<?php%20echo%20admin_url(%27admin-ajax.php%27);%20?>

How fix this problem? Thanks