Pandoc plugin: how to change margins on PDF export

I’m using the “Pandoc Plugin” 0.2.5 to export to PDF. I’d like a wide margin on the right hand side. I thought adding this up front would work:

---
geometry: 
- right=5cm
---

because it does when I run pandoc from the Terminal.

Am I doing this wrong - should I be using a “template” instead?

Thanks