I need to pass in a CRM ID# and retrieve it later. What hooks should I use?

So here's the deal... We're using Appointments+ for internal usage only. We use Highrise CRM. I am comfortable with their API and have written a couple other scripts to integrate it with other tools we use.

I want to pass in the Highrise ID via a GET variable. Once an appointment is made I want my plugin to retrieve that ID along with all of the field data collected by Appointments Plus. At that point my plugin will shove all the information into Highrise.

I'm digging through the code. It looks like my best bet would be "wp_ajax_post_confirmation"?

Also, my plan was to shove the Highrise ID into an additional field. That means my plugin needs to catch the appointment scheduling page as it's being generated. What hook should I use for that?

Once I figure out what hooks to use I'll be fine. It would be really cool if you folks would create some documentation regarding those hooks...