Conditional concatenation in addons

How can I conditionally concatenate evals in Addons?
How can i reference response variables in expressions on Addons?

Here is a picture of the addon:

I have created a full_location eval which is supposed to conditionally format the full location text based on the other values.

For example: only include Country if City or State are missing.

I am trying to use expressions but I am unsure how to reference the fields since the Query All Records response variable is unnamed in Addons.

Thanks in advance