Hook for submit event form?

I’m using the Posts 2 Posts plugin to connect my event post type to a second custom post type. This all works correctly and as expected on the WordPress admin side.

However I need to now update the “Submit an Event” form on the user side to support this new connection. Is there a hook or filter that I can use to run some custom code when the event form is submitted? The custom code basically just loops over an array (a new <select> field that I added to the form) and creates a new connection for each array item.

Thanks.