SOLVED: How to Add a Subdomain to an Active Directory Domain

Here the simple steps to add a subdomain to an on-premises Active Directory domain:

1. Open Active Directory Domains and Trusts: On the domain controller, open the Server Manager and choose Tools > Active Directory Domains and Trusts
2. Add the new UPN suffix: In the Active Directory Domains and Trusts window
   • right-click Active Directory Domains and Trusts
   • choose Properties
   • In the dialog box on the UPN Suffixes tab, type the name of the suffix that you would like to add to your AD forest in the Alternate UPN suffixes box
   • Click the Add and then OK buttons
3. Create a DNS Record for the Subdomain: In your DNS manager, create a new host record for your subdomain.
   • i.e if you want to create a subdomain named “accounts”, you would type “accounts” in the Name field

Note that these steps assume you have the necessary administrative privileges (Domain or Enterprise Admin) to make changes to your Active Directory and DNS settings.