How to open a form in modal from controller while redirect

I can open a form in modal when click on a link (and there are lot of examples in the net as well).

What I like to do: When some one go to a url in browser or redirect to a url, it will open a dialog/modal form.

-I have defined a controller for a url -In the controller function I placed the code like this:

  $response = new AjaxResponse();
  $title = $this->t('Confirmation');
  $form = $this->formBuilder->getForm('Drupal\example\Form\MyModalForm', $entity);
  $response->addCommand(new OpenModalDialogCommand($title, $form, ['width' => '800']));

But its not opening the Modal Form, rather showing json code in a textarea. Surely dialog box library might be missing. So I added the library:

    $form['#attached']['library'][] = 'core/drupal.dialog.ajax';

in the form, in the page preprocess, etc. But none is working.

It is showing json code like this (example)

[{"command":"openDialog","selector":"#drupal-modal","settings":null,"data":"Something to show","dialogOptions":{"width":"800","modal":true,"title":"Confirmation"}}]

Can anyone help me, on how to open the dialog/modal when visiting the url?