Redirect to the last url?

As in Auth0 docs, I can send a state value with the block dialog as here:
but how would I read it back at rails? at callback?

I found this doc: but, its not clear how to read back the state parameter I’ve sent in the callback handler of rails.

The state parameter will be passed back to your callback as a query string, or in the body, depending on the connection you use. You will need to parse this state parameter in your callback, and make the necessary checks.