You can use dotenv package for this purpose.
npm install dotenv --save
After installing the package, add this in the top of your config:
const webpack = require('webpack'); // only add this if you don't have yet
// replace accordingly './.env' with the path of your .env file
require('dotenv').config({ path: './.env' });
then in plugins
section, add this:
new webpack.DefinePlugin({
"process.env": JSON.stringify(process.env),
}),