I understand how to use the endpoint - the question that I have is how pass the cookie ID to the backend. I don't see any call with the munchkin that exposes the ID so that I can then pass it to a backend system.
But maybe I'm thinking about this incorrectly, so instead I'll describe the use case: I've got a form on our website - we create and manage the form (no Marketo). I want to be able to check if the person has a Marketo cookie and lead information, and automatically pre-populate the form. This is the way it works with Marketo landing pages, but unfortunately this is not available in Forms 2.0. As a result, I'm trying to setup the API to both submit the info to Marketo, and to check if a lead exists already and pre-populate.
If it matters, we're using AngularJS on the frontend and then storing the actual code for hitting the REST API in a Node application.