Syntax:

import { useState } from "react";
import ReactDOM from "react-dom/client";

function SignIn(){
  const [ userName, setUserName ] = useState("");

  return (
    <>
      <h1>SignIn</h1>
      <input
        type="text"
        value={userName}
        onChange={(e) => setUserName(e.target.value)}
      />
    </>
  )
}

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<SignIn/>);