I'd recommend to pass user identifier (id, guid or anything else) in more secure way:
You're probably adding React container (root) to the page either through template layout or web part like static html. Last one is where you could use Kentico macro:
<div id="root" user="{%CurrentUser.ID|(identity)GlobalAdministrator%}"></div>
So now you could easily get the value in React root component and pass it through components tree:
const el = document.getElementById('root');
var user = el.getAttribute('user');
ReactDOM.render(<App userId={user} />, el)
This approach gives you cleaner URL and reduces chances someone will play with the value in the URL.