How to enable read/write global shared deploy keys

The documentation says, “Global Shared Deploy keys allow read-only or read-write (if enabled) access to be configured on any repository in the entire GitLab installation.” I was able to create a read-only Global Shared Deploy key, but I don’t see anyway to create one that is read/write.
The wording in the documentation makes me guess that I need to enable their creation somehow, but I can’t figure out how that is enabled.
Does anyone know how to create a Global Shared Deploy key that allows read-write access?

Hello,

The Global Shared Deploy keys functionality is available for self-managed GitLab instances and not available on GitLab.com.

At this time, the option is to do per project deploy keys. If you would like group level deploy keys, we encourage you to file a feature request on our tracker.

I should have been clearer. I am using read-only Global shared deploy keys successfully. I am on self-managed GitLab instance: GitLab Enterprise Edition [11.7.4-ee]
The question is about how to enable the use of read-write Global Shared Deploy keys. I have seen the documentation you linked; that’s where the quote comes from.
It says read-write access can be configured if enabled but I can’t find how to enable it.
Is this the wrong forum for GitLab EE?