get_option from externally available page

I’m building a plugin that has builds a form via widget in the sidebar. I would like to post the results of the form back to the plugin directory to a processing page.

The problem is that the processing page is externally available this way. Form posts to page. Page posts to API of another site. The results are returned and the user returns to the page where the submission occurred.

My page in the plugin’s directory is called ‘subscribe.php’. I want to grab some of the options for the plugin using get_option. I have an admin page that saves these to the wp_options table.

The question: What includes or require_once pages do I need to include on my stand-alone processing page that will allow the page to get the options from the wpdb?