TALES expressions for easyform

I would like to configure an easyform to populate some fields with TALES expressions.
For example, for the "topic" (Subject) field, here's the provided help:

"Default Expression A TALES expression that will be evaluated when the form is displayed to get the field default value. Leave empty if unneeded. Your expression should evaluate as a string. PLEASE NOTE: errors in the evaluation of this expression will cause an error on form display."

I tried "context/Title" and "string:${context/Title}" ; the error I received is "KeyError: 'context'"

My question: How to write valid TALES expression to set the fields of an easyform?

Potentially, you must look at the EasyForm code to find out which "top level variables" are defined. Maybe, reading the documentation or look at the tests may also provide the answer.

The available "top level variables" are not built into TALES but are defined by the specific integration. For example, form might be a good top level variable for a TALES integration into a form framework.

As far as I know you have the fields by its identifier like subject, email and so on as globals for the expression. I am not sure whats else. Probably not much b/c off security reasons?