Replace your captcha markup with one element and one script. Read the signed token on submit and verify it server-side against /.well-known/ — no callback to Manav.
<!-- where your captcha used to be -->
<div class="manav-check" data-endpoint="/labs/captcha/api/index.php"></div>
<script src="/labs/captcha/widget.js"></script>
// on submit, the form carries a hidden "manav-token";
// verify its Ed25519 signature against /.well-known/ on your server.