Join an existing directory domain in macOS Server
When joining an existing directory domain, you need the following information:
Parent Server: this is the name of the Open Directory server that your server is joining as a replica.
Directory administrator credentials: this is the directory administrator user or a user that has directory admin privileges.
You also must enable remote administration access to your server before joining a directory domain.
In the Server app sidebar, select Open Directory.
In the Parent Server field, enter the name of the Open Directory domain that your server is joining.
Enter the name and password for the directory server your server is joining.
Confirm your settings, then click Set Up.