If you are using React Router v4 you should be able to set it using basename={foobar
}.
<Router history={browserHistory} basename={'foobar'}>
<Route path="https://stackoverflow.com/" component={App} />
</Router>
Link to the docs: https://reacttraining.com/react-router/web/api/BrowserRouter
Note: if using create-react-app in a sub directory you will also want to set "homepage": "/foobar/",
in your package.json file. So the production build points at the right path.