It's simple you can Visit this, And save the file as popper.min.js
then import it.
import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';
import 'bootstrap/dist/css/bootstrap.css';
import './index.css';
import 'bootstrap/dist/js/popper.min.js';
global.jQuery = require('jquery');
require('bootstrap');