Simple Hierarchy formula question

Hi everyone.

I'm new to SAC and so still trying to grasp some basics. I'm struggling with what should be a very simple formula.

I have a hierarchy that i want to reference in an "if formula" to enable me to group a few customers together. However i'm unable to find the correct identifier to validate the formula.

The Dimension is "Stock Code" and the parent I want to reference in the formula is "Customer" - I've tried (and failed) with many different variations that seemed logical but none are working.

I've attached a photo of the formula as i've written it, and also the hierarchy that shows the name etc.

Any help would be appreciated