How to add a Hidden Item (Query String Variable)

What is a Hidden Item?

Hidden Items allow you to store data in a response without the respondent seeing it or having to enter it within the survey. Think of them as empty buckets that you can fill with data by adding that data to the survey URL or response session. That data can then be used to identify a response, filter a report, or set a condition in a survey.

For instance, you might want to use the same survey to gather feedback on a series of e