How do I enter contextual filter in views UI so I can use Auto Preview?

I am trying to get Auto Preview to work on views and I can't figure out how to enter the contextual filter so it works.

I have a FPP with an entity reference field. I created a view and added a relationship "Fieldable panel pane referencing Content from FIELD" and then added a contextual filter with FIELD. The view works as expected and gives me the results I require. When I am editing my view, however, I would like to use the auto preview functionality to see results. How do I enter my contextual filter so that auto preview works? I've tried machine name, fpid, URL path, etc. Is there something I'm missing? Any advice would be appreciated, thanks.