Custom Code uses HTTP POST to call the underlying method. GET is not supported on it and you will not be able to call it from browser due to that. However there is a work around to do that using JS SDK. You can write a HTML5 page in which on load event you can use JS sdk to post the data in custom code. You can host this HTML5 page in our CDN using file upload from AppHQ console. (Technical Service > File Upload) and can use this HTML5 page URL in your email.
We are in the process of making Custom code work for GET method and will be available by mid of September release.
Let me know if it helps or you need any additional helps here.