The UPN identifies the domain in which the user account is located. This UPN suffix can be the DNS domain name, the DNS name of any domain in the forest or it can be an alternative domain name as the UPN suffix.
User Principal Name (UPN) suffixes are part of Active Directory (AD) logon names. If you want to add a new UPN suffix, you can follow below steps,
You can use Windows server 2012 and later versions.
To do that, you can go into Active directory Domain and Trust then right click on ADDT and select properties > Select UPN suffix > and enter your new UPN suffix and click add.
Once you done that, close and reopen the ADUC and when you are creating a new user you should be able to select the new UPN suffix for that account.