How can I trigger a loading overlay when I call admin-ajax.php?

I have 100% front-end web app, that calls an ajax modal via admin-ajax.php. The modal can take a bit to load in the form, though, so I'd like to put in a loading overlay while it pulls the data in. How can I do this? I was surprised to find so little info on this when I did a Google search. I'm not a master JS/PHP guy, unfortunately.

Link set up like this: "/wp-admin/admin-ajax.php?action=formcall&form_id=additem"