How to connect React frontend to Flask backend URL in production?

Solution

useEffect(() => {
 fetch("https://backend.herokuapp.com/test", {
  headers: {
    "Content-Type": "application/json",
    "Accept": "application/json",
  },
}).then((response) =>
  response.json().then((data) => {
    setTest(data.tests);
  })
);}, []);

fetch(`${API_URL}/test`, {