Board Thread:Support Requests - Designing Your Wikia/@comment-11008902-20160219181528/@comment-1251315-20160219184150

You weren't here when Wikia had to lockdown the MediaWiki namespace due to vandals. You need to contact Wikia through Special:Contact and request that the JS Review Tool be enabled on the wiki's in question, but you'll have to explain to Wikia why you wish the tool to be enabled (it's not a Wiki Feature, and likely won't be for a very long time).

If it is enabled, you will have to submit javascript posted to your wiki's MediaWiki:Common.js/MediaWiki:Wikia.js files for review. Wikia reserves the right to reject added JS if it violates the customisation policy. See more here: Help:Community CSS and JS

PS: For the use case you have mentioned, you should probably use MediaWiki:ImportJS on the wiki's in question, which does not require the JS Review Tool. See here: Help:Including additional CSS and JS