What's the shortcut syntax to link to another Stack Exchange site? I can't remember and it's not in the help center! But I know it's worked before.
The general form is
sitename is the part of the fully qualified domain name preceding
stackexchange.com. For example,
Some sites have special "magic links", like
[so] (Stack Overflow),
[sf] (Server Fault) and so on.
You can also use links like
[main] to link back and forth between a single site's meta site and main site.
There's more; see Add data.SE style “magic links” to comments for a (hopefully current) list.
Note that these magic links only work in comments. In questions and answers, use the full form.