How to create a variable that lists custom Strings?

Hi there, I'm trying to create a custom list of Strings like 'Full Group' and 'Kleihauer Test' that I can then Direct a custom measures to list next too like so:

Basically my measure would read =(If [Custom List] = "CGS" Then ((Sum ([Test Code]="AA")) - (Sum([Test Code]="A"))), but I don't know how to/if you can create a variable that just list custom Strings

Thanks

Adam