Entra External ID sign-up redirect url with custom domain redirecting back to ciamlogin.com domain

Hey Paul, it sounds like you’ve got your custom login domain working until the post-signup account selection step, where it falls back to <app>. That happens because by default the “issuer” for the account selection flow is still the default identity host. To force it to use your custom domain end-to-end, you need to override the issuer URL in your identity provider configuration (and in your app’s MSAL/OpenID Connect settings) so it points at your custom host instead of the default host.

Here’s what to do:

1. In the Azure portal, go to your App Service (or wherever you configured External ID).
2. Under Authentication → Identity Providers, edit your Microsoft identity provider.
3. In Advanced settings, locate the Issuer URL (it’ll default to https://<your-tenant>.ciamlogin.com/{tenantID}) and change it to your custom domain, e.g.: https://login.contoso.com/{tenantID}
4. Save and let it propagate (takes a couple minutes).

Next, in your application code/config:

• Update your authority to point at the custom domain, for example:

authority: https://login.contoso.com/{tenantID}/v2.0

• In your MSAL (or OpenID Connect) config, include your custom domain in knownAuthorities so MSAL won’t redirect to the default host:

  msalConfig = {

    auth: {

      clientId: "<your-client-id>",

      authority: "https://login.contoso.com/{tenantID}/v2.0",

      knownAuthorities: ["login.contoso.com"],

      redirectUri: "https://login.contoso.com/{tenantID}/authresp"

    }

  }

By overriding the issuer URL and application authority to use your custom hostname, all steps — including the account selection page after the /auth/login/aad/callback response — will stay on your branded domain.

References:

• Custom URL domains in external tenants – Blocking the default domain: https://learn.microsoft.com/entra/external-id/customers/concept-custom-url-domain#blocking-the-default-domain

• Enable custom URL domains for apps in external tenants: https://learn.microsoft.com/entra/external-id/customers/how-to-custom-url-domain#configure-your-applications

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