Subdomain in gitlab

Hello. I’ve my main domain “example.com”. I configured synconization gitlab and active directory with main domain. All users can sing in and work with it.
Now I want to connect gitlab with my sub-domain “city1.example.com”, but don’t understand how to do this.
Anybody can share with me with solutions or give me advices?

Easiest is to start with the Gitlab documentation that explains how to do that: Integrate LDAP with GitLab | GitLab

Thank you.
Right I understand, that I need use to part of with “Use multiple LDAP servers” and my subscription level should “Premiu” or “Ultimate”