Login With AJAX 4.0 introduces support for Gutenberg blocks, including the new Widget blocks area introduced in WordPress 5.8.
The process of adding a widget or block is pretty straight forward. You'd first search for the Login With AJAX block:
When you add the block, you will see the login form by default. All the options available to you are covered in the login form documentation page.
Additionally, you will see an extra option called
Guest Preview Mode or just
Preview Mode in the block toolbar, as per the screenshot below. By toggling this option on/off, you will see the form whilst logged in or logged out.