Hello,
I was also interested in this feature, to make it work I had to create the variable in:
> Actions -> Variables -> Created by you.
Once you create a variable there, it gives you a "Variable ID" which is the identifier you can send in your request.
For example:
I created this draft variable:
I'm sending it like this:

And I'm able to use it in a condition as a Session variable:

or even in the "Assistant says" input:
