How to get domain out of FQDN (for login)?

Hi

I'm confused about FQDN...

I have FQDN like siteA.myCompany.com. In my SQL Server I choose a user from this domain (let's say userA\siteA.myCompany.com). However, SQL Server cuts off the myCompany.com part and adds the user as userA\siteA in its security store.

Now I tried to access my database as userA\siteA.myCompany.com. However, SQL Server doesn't know this user. If I try with userA\siteA everything works as expected.

Can the SQL Server safely get rid of myCompany.com? If I need a user like domain\user can I always cut of the part after the first dot for the domain part?

Thanks for helping me!