How can I change the appearance of the form while it is being submitted

[JonathanHolvey]

When submitting my form, I'd like to be able to add a 'working' indicator, and maybe disable the submit button so that the user knows the form is being submitted.

I had expected BoltForms to add a class to the <form> or submit <button> elements, which could be targeted with CSS, but nothing actually happens until the AJAX request is completed, or the page is redirected.

I've looked at the htmlPreSubmit and htmlPostSubmit options, but these only add static HTML on the page with and without the form respectively.

Is there currently any way to achieve this?

I'm using BoltForms 4.2.3 and Bolt 3.5.