vCenter Single Sign by default requires the user to specify the domain during authentication with vCenter.
Example: JERMSMIT\admin or admin@JERMSMIT.LAB.

You can eliminate the need to insert the domain in the username by following the following steps.

 

  1. Log in to the vSphere Web Client as administrator@vsphere.local or as another user with vCenter Single Sign-On administrator privileges.

  2. Browse to Administration > Single Sign-On > Configuration.
  3. Under the Administration, configuration locate the Identity Sources tab
  4. On the Identity Sources tab, select an identity source and click the Set as Default Domain icon.
  5. In the domain display, the default domain shows (default) in the Domain column. Set the domain of choice as your new default.

The next time when you attempt to login into vCenter, you can omit the DOMAIN from your username.

Full ref located here
Full Link: https://docs.vmware.com/en/VMware-vSphere/6.5/com.vmware.psc.doc/GUID-11E651EF-4503-43BC-91F1-15502D586DE2.html