:with attribute for form_remote_tag

The :with attribute is very crappily documented. It works for me with
link_to_remote, but I can’t seem to get it to work with form_remote_tag.

Does anyone know whether it is possible, at all, to get :with working
with form_remote_tag?

For future reference, how am I supposed to determine whether or not a
particular attribute is supported by a particular helper? The docs for
link_to_remote don’t seem to indicate that :with is supported, but it
clearly works.

Nobody can shed some light on this? It would be nice if I could get this