How to generate external secure form for clients in Rails


I have an application where businesses can create accounts and add
information about their clients. I would like to be able to generate a
form that they can put on their website so that clients can
automatically enter their info into the business's account. I know I
might be able to do something like:

   <% form_tag my_site_url_with_action do %>
   ....put here the fields for client info

But I was wondering if there is a way to make it secure by ensure that
it's coming from my client's website as opposed from some spam website
or so. Also I was wondering whats the best way to make it AJAX (with
minimal inclusion of Rails JS for AJAX)

Any ideas?