Testing nested event properties

As long as you're referencing the nested event property in the template, once you go to "Preview & Test" you could enter a value to test. The easiest way to reference a nested property is shown on the screenshot below. 

You could also just reference the main property of the event that contains the nested object and paste the object value in the "Preview & Test" section's variable. This is if you need to confirm how it renders and how you can extract data from the object.