Chiedo un ulteriore suggerimento per poter gestire la classe modal con una form che richiama un'altra form... di seguito l'esempio:
FORM: modal.html
<!DOCTYPE html>
<html lang="en">
<head>
<title>Modal.html</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="assets/css/bootstrap.min.css">
<script src="./assets/js/bootstrap.min.js"></script>
<script src="./assets/js/vendor/jquery-1.4.2.min.js"></script>


</head>
<body>

<div class="container">
<h2>Basic Modal Example</h2>
<!-- Trigger the modal with a button -->
<button type="button" class="btn btn-info btn-lg" data-bs-toggle="modal" data-bs-target="#myModal"><a href="modal-f.html">Open Modal</a></button>


</div>

</body>
</html>

Form: Modal-f.html
<!DOCTYPE html>
<html lang="en">
<head>
<title>Modal.html</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="assets/css/bootstrap.min.css">
<script src="./assets/js/bootstrap.min.js"></script>
<script src="./assets/js/vendor/jquery-1.4.2.min.js"></script>
</head>

<body>
<!-- Modal -->
<div class="modal fade" id="myModal" role="dialog">
<div class="modal-dialog">

<!-- Modal content-->
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal">&times;</button>
<h4 class="modal-title">Modal Header</h4>
</div>
<div class="modal-body">
<p>Some text in the modal.</p>
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-bs-dismiss="modal">Close</button>
</div>
</div>

</div>
</div>
</body>
</html>