It used to be two open brackets c: name of other wikia close brackets. That doesn't work now that instead of wikia there is fandom in the address. Is there a new command for easily linking wikias/fandoms?
Yeah, the interwiki linking behavior changed early September at the latest. That is when I noticed "c:" wasn't cutting it anymore. Now, it has to be an interwiki prefix. As I learned here, apparently neither the list returned by api.php nor the one at MediaWiki:Interwiki_map are used as the actual map. So basically, while you can use those to get an idea of what might be interwiki prefixes, they aren't a guarentee.
I am curious as to why they made this change. I suppose it might have to do with redirects. While playing around with the old system, I found that you could basically bounce back-and-forth between communities. See here. Perhaps they wanted to remove the chance of someone giving a link that redirects for an insane number of times before finally landing on an actual page.