If you have different log in pages for this users, you can create a bookmark application (or any other sort of applications inside Okta) to redirect the users there. Once this is done, in the call to
/api/v1/authn, you will need to send relayState parameter for Okta to redirect the user there after authentication. You can find more details about this parameter and examples here.
If you want to have the users authenticate on the same log in page, then the best solution would be to get them authorized against an OIDC app that generates an id_token with group membership (doc here). Once your app receives the JWT token, you can decode it, review the group membership and, based on that, redirect the users to the specific URLs.