const MyModalDialog = BootstrapModal.extend({
    id: "my-modal-dialog",
    initialize : function() {
      ...
    },
    ...
  });

  function showMyDialog(ev) {
    ev.preventDefault();
    const chatView = getChatViewFromElement(ev.currentTarget);
    const myDialogModel = new Model();
    _converse.api.modal.show(MyModalDialog, {
      'model': myDialogModel,
      'chatroomview': chatView
    }, ev);
  }