Related to Auth0 and react-auth0 sdk To add a custom parameter to the authentication endpoint of which auth0 interacts we can pass it like this in the object

authorizationParams: {
          screen_hint: "signup",
          ui_locales: "en",
          custom_param: "custom_param",
        },

authorizationParams object is used in methods loginWithRedirect, withAuthenticationRequired provided by @auth0/auth0-react

once the custom parameter is passed we can see it in /authorize endpoint when we open the Universal login or signup form from Auth0

If we want to take any custom actions based on this custom parameter we can access it in Auth0 post login custom actions