Using Plugin for External Form

Complicated, but possible. There are several approaches to take, but they all depend on getting the form submission with this other form to send the data to the plugin. Probably the easiest would be to attach javascript to this other form and do an AJAX call to a script in WP that puts the submission data into the database. If you study the main script in the plugin, you’ll find a method “process_form_submission()” that you can use, or you can write to the database directly using $wpdb.

If that doesn’t make sense to you, you’ll probably have to pull in a developer to do the coding.