How do I access the appState I set in loginWithRedirect in a React app?

I have a react app which allows the user to save documents. If the user is not logged in and tries to save it, I want the user to be redirected to a login prompt, and then be sent back to the app, which will then load the unsaved project and show the save dialog. I currently have this code connected to the save button:

const { isAuthenticated, loginWithRedirect } = useAuth0();
async function save() {
  if(!isAuthenticated) {
    loginWithRedirect({
      appState: "insert current document here"
    });
  }
  else {
    // Save the project to a database
  }
}

How can I access the appState once Auth0 redirects the user back to my app?