So how can I access and modify callback URL from Login customization script?
Sorry, I misread the initial scenario. My thought was to add the parameter in the authorize call that you’re making from your client application (as opposed to the login page). I see that you wanted to have the switch in the Auth0 Login Page. In this case, it’s afaik not possible with the out of the box Login widget (Lock) that Auth0 provides on the login page.