How to use fetch correctly?

I'm working on my application now. I'm trying to use fetch for login page, but I don't really understand how to use fetch even reading some example of code. Could anyone please help me get that?

For instance, I have to use these information to login to my server. username: "user" password: "1234"

then I want server return that login success or not and return a token if loging in is success

I tried to used this code

render() {
return (
  fetch('mysite', {
    method: 'POST',
    body: JSON.stringify({
      username: "user",
      password: "1234",
    })
  })
  .then((response) => response.json())
  .then((responseJson) => { return responseJson.success; })
  .catch((error) => {
    console.error(error); })
);
}

I don't know how to return success and token information. Is there are a header of this fetch? Actually I don't know that how to place this code. Is that true to use this in return of render section?

Thank you for recommendation.