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 sidebar of the Server app  , select Open Directory. , 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. 
- Click Next. 
- Confirm your settings, then click Set Up.