Wordpress - REST API: Backbone and custom endpoint

It looks like you are trying to send a POST request to the /istermactive endpoint, is that correct? (I think you may want to remove the trailing slash from the endpoint?)

I'm not really sure the wp-api client is the right tool for a standard ajax POST, you may want to use jQuery.ajax or use WordPress's helper wp-ajax.

The wp-api client is designed to help you when you want to interact with a collection of items or a single item (eg posts or a post) when retrieved from the WP-API. It creates Backbone models and collections by parsing the API schema. Check here for some docs (which are in need of some updates).

previous answer:

It appears your JS isn't loaded. You have registered the script, but haven't enqueued it. Add wp_enqueue_script after you register it. You can also use wp_localize_script to pass dependencies to your script and make variables from your php available to it.

Both above linked to codex on those functions.