Per-request settings - how to change previewOption for individual requests

[MarkDennerFC]

The last section of the documentation implies that I can change various settings on a pre-request basis, but I can't figure out how. I want to be able to switch between previewOption values (exchange if I'm getting an error, body if I just want the output, etc.). I've tried this:
# @previewOption exchange
but it doesn't work. What am I doing wrong?

Thank you!!!! This extension is brilliant and solves a lot of challenges for me!