Overview
This article explains how custom parameters can be passed from the application and then read in Actions.
Solution
The applications can send the custom parameters as a query parameter with the /authorize call while starting authenticating flows that use the universal login or can append them to the /oauth/token calls for the refresh token and resource owner flows.
The method of appending the custom parameters may change between different Auth0 SDKs. For single-page applications using the auth0-spa-js library, the parameter needs to be appended to the AuthorizationParams object. React, and Vue SDKs have the same capability using a similar object with the same name.
Once the custom parameters are appended, they can be accessed in the Action via event.request.query attribute.