How to inject API server URL when deploying react frontend?

You can try this:

// http.js
const getBaseUrl = () => {
  let url;
  switch(process.env.NODE_ENV) {
    case 'production':
      url="https://stackoverflow.com";
      break;
    case 'development':
    default:
      url="https://google.com";
  }

  return url;
}

export default axios.create({
  baseURL: getBaseUrl(),
});