Hi @Jörg Lang
Thanks for reaching out to Microsoft Q&A.
Unfortunately, Entra ID will use the UPN attribute synced from the On Premises AD, and if that's not a routable domain, it will use the company.onmicrosoft.com domain for Entra ID sign in. The documentation below details that as you may already know:
I'm not sure if that's your main goal, but if you need your users to sign in with a different username, you could test and use Alternate ID feature that is currently in Preview now. There are some prerequisites and limitations to use it, but depending on your need it might be an option for you.
Thanks,
Fabio