I came here to ask about how to do it until I figured it out, so I thought I'd share.
From Zotero's website
You can edit most Zotero preferences through the Preferences window in Zotero"
To view the the full list of Zotero's preferences, including many hidden preferences, go to the Advanced pane of the Zotero preferences and click “Config Editor”.
When you open it, there's no option to simply "create" a new key. You need to first search for something (in this case, what's listed here - extensions.zotero.pmcid.auto). When you search for it, it will not be found. However, you will see an option to add it (plus sign on the right side). You will also see options to choose what kind of "key" (?) it will be - by default for me it was on boolean, which is what's needed according to the instructions. Then it was already set as "true".
That was that.