Can the built-in "No account? Create one" link redirect to a custom sign-up page?

With the current documented External ID feature set, that built-in link’s destination is not configurable in a browser-delegated sign-up/sign-in user flow. The page is Microsoft-hosted, and the user flow controls the transition from sign-in to its Entra sign-up step. Company branding, CSS, localized strings, page layout, and custom URL domains can change presentation or the authentication host, but not the link target.

If registration must happen on your page, add a separate Register action in the application before starting the Entra authorization request. Then disable sign-up in the user flow so users do not see a second registration route; Microsoft documents this through the user flow’s isSignUpAllowed setting.

If you need Entra-managed sign-up with an app-owned UI, evaluate native authentication where your platform supports it. It uses Microsoft’s SDK/API while your application owns the screens and flow handling.