How to allow API into the web deploy

I am wondering when i want to execute the app in web, i only have this problem, how i can add the corse into the header? i try to add into my API code these lines of code but nothing succeded? what i can do? it is not the only API i have to call but this block all the execution

 res.setHeader("Access-Control-Allow-Origin", "https://beeto.flutterflow.app");
  res.header(
    "Access-Control-Allow-Headers",
    "Origin, X-Requested-With, Content-Type, Accept"
  );