const { useRef } = React;

const Form = () => {
  const inputRef = useRef();
  
  const handleClick = () => {
    inputRef.current.focus();
  };

  return (
    <>
      <input ref={inputRef}/>
      <button onClick={handleClick}>Focus</button>
    </>
  );
};

ReactDOM.render(<Form />, document.getElementById('app'));