The best way to access custom attributes from an event object is by using HTML data-* attributes and retrieving them with event.target.getAttribute().
Example:
function App() {
  const handleClick = (event) => {
    const customValue = event.target.getAttribute("data-id");
    console.log("Custom Attribute:", customValue);
  };
  return <button data-id="12345" onClick={handleClick}>Click Me</button>;
}
export default App;