Show custom prompt/modal on window/tab closed

Question

How do I deal with this part of recruitment task?

"The game saves when the tab/window is closed and a prompt appears to inform the player about this."

beforeunload event is not working well. It only shows the default message.

class App extends React.Component {
  onUnload = e => { 
    e.preventDefault();
    e.returnValue = '';
 }  

componentDidMount() {
  window.addEventListener("beforeunload", this.onUnload);
 }

componentWillUnmount() {
  window.removeEventListener("beforeunload", this.onUnload);
 }

render(){
  return (<div>App</div>)
  }
}

export default App;