Hi @dy.le, and welcome to the Auth0 Community!
Unfortunately, it is not currently possible to add a custom message to the enrollment flow without redirecting the user to a custom page. If you do consider redirecting the user, though, a post-login action that checks if the user has not yet been enrolled (using a flag in app_metadata) would be the best way to do it.
I hope this helps you, and don’t hesitate to ask more if something is unclear!
Sincerely,
Teodor.