How do I attach data for a temporary event?

I am using Heroku Temporize add-on to schedule tasks. temporarily document :

Any message data provided with a request contains data about the state your application uses to decide how to handle the event. For example, the Data field can contain the account number for a user on your system, or a piece of data to process.

I've tried posting text / plain, url encoded and JSON data, but never get any data on my endpoint.