Creating a filter by county within Looker Studio

Hello!

I have been trying to create a filter within looker studio to only see cities within a specific county (in California for example) for a geo map that shows active users per city.

I have been trying to blend data, but I am new to looker studio so I am not sure how to create a filter that would include specific cities if someone can help! I don’t want to have to do a filter for every single city!