How to call react express api on production

[shinnachot]

From this react-web-express-api-development-production article when i want to call api on server side in localhost i call "localhost:3001/api/hello" but how to call this api on production if i deploy real production like "test.com" and i try to call api like this "test.com/api/hello" it's not working they show display webpage by default