Where do I set backend url for production?

[RasmusN]

I am trying to deploy my website on AWS S3 as a static website.
I'm running into problem because I can't find where to define the API_BASE_URL for axios post/get requests to my backend.
The instructions for defining this on the dev server is clear:
API_BASE_URL=http://custom.backend:8000 yarn dev

As far as I can see, the API_BASE_URL is only used for testing. Where do I set the backend url for production build?