How can I attach to a stateless component’s ref in React?

You can useuseRef hook which is available since v16.7.0-alpha.

EDIT: You’re encouraged to use Hooks in production as of 16.8.0 release!

Hooks enable you to maintain state and handle side effects in functional components.

function TextInputWithFocusButton() {
  const inputEl = useRef(null);
  const onButtonClick = () => {
    // `current` points to the mounted text input element
    inputEl.current.focus();
  };
  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

Read more in Hooks API documentation