Simply add the script to your page, you can download it from here
<script src="modal-close-on-back.min.js"></script>
Or CDN:
<script src="https://cdn.jsdelivr.net/gh/FranBar1966/bootstrap-5-modal-close-on-back@master/src/modal-close-on-back.min.js"></script>
No further action is required.
See: Example
It can be used in combination with: modal dynamic
<button
type="button"
class="btn btn-primary"
data-bs-toggle="modal"
data-bs-target="#exampleModal1"
>
Simple default modal
</button>
<div class="modal fade" id="exampleModal1" tabindex="-1" aria-labelledby="exampleModal1Label" aria-hidden="true">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<h1 class="modal-title fs-5" id="exampleModal1Label">Modal title</h1>
<button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
</div>
<div class="modal-body">
You can click the Back Button to close this modal.
</div>
<div class="modal-footer">
<button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Close</button>
</div>
</div>
</div>
</div>