React Native + Expo, help with storing + accessing API key

I am coding an app in React Native (+Expo). But I need a way to store my API-key safely, so that it won't be uploaded to my Git-repo (but easy for someone else to add their own if they want to test the code on their own).
I have tried a number of ways, but without success. I did try adding the key in a .env file, and also another solution where I stored it in "extra" in app.json. None of the solutions worked.