Hi, you can add the target link via the inbuilt fields in the menu in the dashboard when adding a custom link.
To add the attribute data-toggle="modal"
follow this guide https://wpbeaches.com/adding-attribute-wordpress-menu-item/
For your target modal just add the code and content from the markup –
<!-- Modal -->
<div id="myModal1" class="modal fade" tabindex="-1">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
<button class="close" type="button" data-dismiss="modal">×</button>
<h4 class="modal-title">My Title in a Modal Window</h4>
</div>
<div class="modal-body">This is the body of a modal...</div>
<div class="modal-footer">This is the footer of a modal...</div>
</div><!-- /.modal-content -->
</div><!-- /.modal-dialog -->
</div><!-- /.modal -->