I tried VisualEditor earlier this evening. I was trying to create a link to "wikidata:Wikidata:Showcase items" from a page on the English Wikipedia. It seems that adding (certain types of?) interwiki links is not possible with VisualEditor currently. Every attempt I made with the link editor to add a link to "wikidata:Wikidata:Showcase items" failed miserably.
T51316: VisualEditor: It should be possible to create interwiki links and edit them subsequently says this issue is fixed, so I'm not really sure what's going on here.