successMessage only accepts a String so adding a html tag would only not render it as HTML properly and rather display as plaintext only.
t’s not easy to implement it the way you want but you can “hack” the page with javascript and modify the content during runtime (DOM Manipulation). If it is to redirect users back to your app, you can use the Redirect To field in Password reset email template: https://auth0.com/docs/email/templates#change-password-confirmation-email