Board Thread:Support Requests - Getting Started/@comment-47051193-20201004002405/@comment-9605025-20201004003425

What do you mean by "add"? If you mean in wiki content, see Help:Interwiki_link. If you mean in Discussions and other Feeds-based discussion features, you need to use the full URL just like everything else. If you mean adding interwiki prefixes, you need to send a request to staff. If you mean adding interlanguage prefixes, you need to submit a request (here).