Create Ref using React.createRef without using constructor in React?

Yes, you can. For example:

const MyComponent = () => {
  const inputRef = React.createRef();

  return (
    <input ref={inputRef} />
  );
}

The only thing you cannot do, is pass the ref attribute to a functional component:

render() {
  // This won't work.
  return <MyFunctionalComponent ref={this.inputRef} />
}

More info from the official docs, here:

You can, however, use the ref attribute inside a function component as long as you refer to a DOM element or a class component:

You can declare it as an class field just like state.

class App extends React.Component {
  state = { counter: 0 };
  inputRef = React.createRef();
}

Babel will transpile it into something like the code below in stage-2 presets.

constructor(props) {
    super(props);

    this.state = { counter: 0 };
    this.inputRef = React.createRef();
  }

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