(Securely) registering a new user via JSON API with a custom Zapier action

I want to use Zapier to register a new Wordpress user — the trigger will be a new purchase on Gumroad.

This plugin — http://ift.tt/1reRPhF — allows you to register users using a public JSON interface, eg by hitting http://ift.tt/1ybHXpp

I'm new to all this, but know that I can get Zapier to insert the purchaser's email from Gumroad into {{email}}.

My question is, how can I do this in a secure way?

(At the moment anyone can hit http://ift.tt/1D14Y5Y and get the key they need)

And maybe it can even be done without using Zapier?