Configure yaml file for circle CI including envionment variables for an Angular project

I have a project I'm trying to build, but my .api-keys document is being gitignored.

So, I added my keys as environment variables to the project on circle CI.

My problem is I'm not quite sure where/how to let my yaml config script know what they are:

old config script:

version: 2.1
orbs:
  cypress: cypress-io/cypress@1.0.1
workflows:
  build:
    jobs:
      - cypress/install:
          build: 'npm run build'
      - cypress/run:
          requires:
            - cypress/install
          start: 'npm start'

Line I'd like to add (I think?):

environment: 
    masterFirebaseConfig: $masterFirebaseConfig

Is this the correct thing to do? Where should this line go in the yaml above?

Many thanks for any tips!

from Configure yaml file for circle CI including envionment variables for an Angular project