Board Thread:Support Requests - Getting Technical/@comment-34945554-20200722104438/@comment-168424-20200723225438

It looks like you have to explicitly request custom JS be enabled on your wiki from FANDOM staff, if you have never had it enabled before, but after that you should be able to edit MediaWiki:Common.js.

It also might be that, if you had custom JS enabled, but didn't get around to doing anything, they disabled it, because I swear some wikis I admin had it enabled, but now it isn't.